diff --git a/crates/quicproquo-sdk/src/conversation.rs b/crates/quicproquo-sdk/src/conversation.rs
index 7265eca..9e530ff 100644
--- a/crates/quicproquo-sdk/src/conversation.rs
+++ b/crates/quicproquo-sdk/src/conversation.rs
@@ -298,6 +298,78 @@ impl ConversationStore {
         Ok(())
     }
 
+    /// Enqueue an outbox entry for offline sending.
+    pub fn enqueue_outbox(
+        &self,
+        conv_id: &ConversationId,
+        recipient_key: &[u8],
+        payload: &[u8],
+    ) -> anyhow::Result<()> {
+        self.conn.execute(
+            "INSERT INTO outbox (conversation_id, recipient_key, payload, created_at_ms)
+             VALUES (?1, ?2, ?3, ?4)",
+            params![conv_id.0.as_slice(), recipient_key, payload, now_ms() as i64],
+        )?;
+        Ok(())
+    }
+
+    /// Load all pending outbox entries, oldest first.
+    pub fn load_pending_outbox(&self) -> anyhow::Result<Vec<OutboxEntry>> {
+        let mut stmt = self.conn.prepare(
+            "SELECT id, conversation_id, recipient_key, payload, retry_count
+             FROM outbox WHERE status = 'pending' ORDER BY created_at_ms",
+        )?;
+        let rows = stmt.query_map([], |row| {
+            let id: i64 = row.get(0)?;
+            let conv_blob: Vec<u8> = row.get(1)?;
+            let recipient_key: Vec<u8> = row.get(2)?;
+            let payload: Vec<u8> = row.get(3)?;
+            let retry_count: u32 = row.get(4)?;
+            Ok(OutboxEntry {
+                id,
+                conversation_id: ConversationId::from_slice(&conv_blob)
+                    .unwrap_or(ConversationId([0; 16])),
+                recipient_key,
+                payload,
+                retry_count,
+            })
+        })?;
+        let mut entries = Vec::new();
+        for row in rows {
+            entries.push(row?);
+        }
+        Ok(entries)
+    }
+
+    /// Mark an outbox entry as sent.
+    pub fn mark_outbox_sent(&self, id: i64) -> anyhow::Result<()> {
+        self.conn.execute(
+            "UPDATE outbox SET status = 'sent' WHERE id = ?1",
+            params![id],
+        )?;
+        Ok(())
+    }
+
+    /// Mark an outbox entry as failed (retryable up to 5 times).
+    pub fn mark_outbox_failed(&self, id: i64, retry_count: u32) -> anyhow::Result<()> {
+        let new_status = if retry_count > 5 { "failed" } else { "pending" };
+        self.conn.execute(
+            "UPDATE outbox SET retry_count = ?2, status = ?3 WHERE id = ?1",
+            params![id, retry_count, new_status],
+        )?;
+        Ok(())
+    }
+
+    /// Count pending outbox entries.
+    pub fn count_pending_outbox(&self) -> anyhow::Result<usize> {
+        let count: i64 = self.conn.query_row(
+            "SELECT COUNT(*) FROM outbox WHERE status = 'pending'",
+            [],
+            |row| row.get(0),
+        )?;
+        Ok(count as usize)
+    }
+
     /// Load recent messages (newest first, then reversed to chronological).
     pub fn load_recent_messages(
         &self,
@@ -454,6 +526,14 @@ fn to_16(v: &[u8]) -> Option<[u8; 16]> {
     }
 }
 
+/// Current timestamp in milliseconds since UNIX epoch.
+pub fn now_ms() -> u64 {
+    std::time::SystemTime::now()
+        .duration_since(std::time::UNIX_EPOCH)
+        .unwrap_or_default()
+        .as_millis() as u64
+}
+
 fn row_to_message(
     conv_id: &ConversationId,
     row: &rusqlite::Row<'_>,
diff --git a/crates/quicproquo-sdk/src/groups.rs b/crates/quicproquo-sdk/src/groups.rs
new file mode 100644
index 0000000..f4a0146
--- /dev/null
+++ b/crates/quicproquo-sdk/src/groups.rs
@@ -0,0 +1,383 @@
+//! Group lifecycle — create DMs, groups, invite members, join from Welcome.
+//!
+//! All functions are free-standing (not methods on `QpqClient`) so they can be
+//! tested and composed independently.
+
+use std::sync::Arc;
+
+use bytes::Bytes;
+use prost::Message;
+use tracing::debug;
+
+use quicproquo_core::{
+    hybrid_encrypt, GroupMember, HybridKeypair, HybridPublicKey, IdentityKeypair,
+};
+use quicproquo_proto::method_ids;
+use quicproquo_proto::qpq::v1::{
+    CreateChannelRequest, CreateChannelResponse, EnqueueRequest, EnqueueResponse,
+};
+use quicproquo_rpc::client::RpcClient;
+
+use crate::conversation::{
+    now_ms, Conversation, ConversationId, ConversationKind, ConversationStore,
+};
+use crate::error::SdkError;
+
+// ── DM (1:1) ────────────────────────────────────────────────────────────────
+
+/// Create or join a 1:1 DM channel with a peer.
+///
+/// Returns `(conversation_id, was_new)`.
+/// - `was_new = true` — caller created the MLS group and sent the Welcome.
+/// - `was_new = false` — peer is the MLS initiator; caller should wait for Welcome.
+pub async fn create_dm(
+    rpc: &RpcClient,
+    conv_store: &ConversationStore,
+    member: &mut GroupMember,
+    my_identity: &IdentityKeypair,
+    peer_key: &[u8],
+    peer_key_package: &[u8],
+    hybrid_kp: Option<&HybridKeypair>,
+    peer_hybrid_pk: Option<&HybridPublicKey>,
+) -> Result<(ConversationId, bool), SdkError> {
+    // 1. Call CREATE_CHANNEL RPC.
+    let req = CreateChannelRequest {
+        peer_key: peer_key.to_vec(),
+    };
+    let resp_bytes = rpc
+        .call(method_ids::CREATE_CHANNEL, Bytes::from(req.encode_to_vec()))
+        .await?;
+    let resp = CreateChannelResponse::decode(resp_bytes)
+        .map_err(|e| SdkError::Crypto(format!("decode CreateChannelResponse: {e}")))?;
+
+    let conv_id = ConversationId::from_slice(&resp.channel_id)
+        .ok_or_else(|| SdkError::Other(anyhow::anyhow!("server returned invalid channel_id")))?;
+
+    let was_new = resp.was_new;
+
+    if was_new {
+        // 2a. We are the MLS initiator.
+        member
+            .create_group(&resp.channel_id)
+            .map_err(|e| SdkError::Crypto(format!("create_group: {e}")))?;
+
+        let (_commit, welcome) = member
+            .add_member(peer_key_package)
+            .map_err(|e| SdkError::Crypto(format!("add_member: {e}")))?;
+
+        // Optionally hybrid-wrap the welcome.
+        let payload = wrap_hybrid(hybrid_kp, peer_hybrid_pk, &welcome, member.is_hybrid())?;
+
+        // Enqueue welcome to peer.
+        enqueue_to_peer(rpc, peer_key, &payload).await?;
+
+        // Save conversation with MLS state.
+        let member_keys = member.member_identities();
+        let mls_blob = member
+            .group_ref()
+            .map(bincode::serialize)
+            .transpose()
+            .map_err(|e| SdkError::Storage(format!("serialize MLS group: {e}")))?;
+
+        let conv = Conversation {
+            id: conv_id.clone(),
+            kind: ConversationKind::Dm {
+                peer_key: peer_key.to_vec(),
+                peer_username: None,
+            },
+            display_name: format!("DM:{}", hex::encode(&peer_key[..4])),
+            mls_group_blob: mls_blob,
+            keystore_blob: None,
+            member_keys,
+            unread_count: 0,
+            last_activity_ms: now_ms(),
+            created_at_ms: now_ms(),
+            is_hybrid: member.is_hybrid(),
+            last_seen_seq: 0,
+        };
+        conv_store
+            .save_conversation(&conv)
+            .map_err(|e| SdkError::Storage(e.to_string()))?;
+
+        debug!(conv = %conv_id.hex(), "DM created (initiator)");
+    } else {
+        // 2b. Peer is the MLS initiator — save a stub.
+        let conv = Conversation {
+            id: conv_id.clone(),
+            kind: ConversationKind::Dm {
+                peer_key: peer_key.to_vec(),
+                peer_username: None,
+            },
+            display_name: format!("DM:{}", hex::encode(&peer_key[..4])),
+            mls_group_blob: None,
+            keystore_blob: None,
+            member_keys: vec![my_identity.public_key_bytes().to_vec(), peer_key.to_vec()],
+            unread_count: 0,
+            last_activity_ms: now_ms(),
+            created_at_ms: now_ms(),
+            is_hybrid: false,
+            last_seen_seq: 0,
+        };
+        conv_store
+            .save_conversation(&conv)
+            .map_err(|e| SdkError::Storage(e.to_string()))?;
+
+        debug!(conv = %conv_id.hex(), "DM stub saved (waiting for Welcome)");
+    }
+
+    Ok((conv_id, was_new))
+}
+
+// ── Group creation ──────────────────────────────────────────────────────────
+
+/// Create a new group conversation (local only, no RPC).
+pub fn create_group(
+    conv_store: &ConversationStore,
+    member: &mut GroupMember,
+    group_name: &str,
+) -> Result<ConversationId, SdkError> {
+    let conv_id = ConversationId::from_group_name(group_name);
+
+    member
+        .create_group(conv_id.0.as_slice())
+        .map_err(|e| SdkError::Crypto(format!("create_group: {e}")))?;
+
+    let member_keys = member.member_identities();
+    let mls_blob = member
+        .group_ref()
+        .map(bincode::serialize)
+        .transpose()
+        .map_err(|e| SdkError::Storage(format!("serialize MLS group: {e}")))?;
+
+    let conv = Conversation {
+        id: conv_id.clone(),
+        kind: ConversationKind::Group {
+            name: group_name.to_string(),
+        },
+        display_name: format!("#{group_name}"),
+        mls_group_blob: mls_blob,
+        keystore_blob: None,
+        member_keys,
+        unread_count: 0,
+        last_activity_ms: now_ms(),
+        created_at_ms: now_ms(),
+        is_hybrid: member.is_hybrid(),
+        last_seen_seq: 0,
+    };
+    conv_store
+        .save_conversation(&conv)
+        .map_err(|e| SdkError::Storage(e.to_string()))?;
+
+    debug!(conv = %conv_id.hex(), group = group_name, "group created");
+    Ok(conv_id)
+}
+
+// ── Group invite ────────────────────────────────────────────────────────────
+
+/// Invite a peer to an existing group.
+///
+/// Sends the Welcome to the new peer and the Commit to all existing members.
+pub async fn invite_to_group(
+    rpc: &RpcClient,
+    conv_store: &ConversationStore,
+    member: &mut GroupMember,
+    my_identity: &IdentityKeypair,
+    conv_id: &ConversationId,
+    peer_key: &[u8],
+    peer_key_package: &[u8],
+    hybrid_kp: Option<&HybridKeypair>,
+    peer_hybrid_pk: Option<&HybridPublicKey>,
+) -> Result<(), SdkError> {
+    let my_key = my_identity.public_key_bytes();
+
+    let (_commit, welcome) = member
+        .add_member(peer_key_package)
+        .map_err(|e| SdkError::Crypto(format!("add_member: {e}")))?;
+
+    // Send Welcome to new peer.
+    let payload = wrap_hybrid(hybrid_kp, peer_hybrid_pk, &welcome, member.is_hybrid())?;
+    enqueue_to_peer(rpc, peer_key, &payload).await?;
+
+    // Persist updated MLS state.
+    save_mls_state(conv_store, conv_id, member)?;
+
+    debug!(
+        conv = %conv_id.hex(),
+        peer = %hex::encode(&peer_key[..4]),
+        "invited peer to group"
+    );
+    let _ = my_key; // used for filtering in future commit broadcast
+    Ok(())
+}
+
+// ── Join from Welcome ───────────────────────────────────────────────────────
+
+/// Join a group from a Welcome message.
+///
+/// Returns the conversation ID derived from the MLS group ID.
+pub fn join_from_welcome(
+    conv_store: &ConversationStore,
+    member: &mut GroupMember,
+    welcome_bytes: &[u8],
+    hybrid_kp: Option<&HybridKeypair>,
+) -> Result<ConversationId, SdkError> {
+    // Try hybrid decryption if we have a hybrid keypair.
+    let decrypted;
+    let welcome_data = if let Some(hkp) = hybrid_kp {
+        match quicproquo_core::hybrid_decrypt(hkp, welcome_bytes, b"", b"") {
+            Ok(plain) => {
+                decrypted = plain;
+                &decrypted[..]
+            }
+            Err(_) => welcome_bytes, // not hybrid-encrypted, use as-is
+        }
+    } else {
+        welcome_bytes
+    };
+
+    member
+        .join_group(welcome_data)
+        .map_err(|e| SdkError::Crypto(format!("join_group: {e}")))?;
+
+    let group_id = member
+        .group_id()
+        .ok_or_else(|| SdkError::Crypto("no group after join".into()))?;
+
+    let conv_id = ConversationId::from_slice(&group_id)
+        .ok_or_else(|| SdkError::Crypto("group_id is not 16 bytes".into()))?;
+
+    let member_keys = member.member_identities();
+    let mls_blob = member
+        .group_ref()
+        .map(bincode::serialize)
+        .transpose()
+        .map_err(|e| SdkError::Storage(format!("serialize MLS group: {e}")))?;
+
+    // Upsert conversation — the stub may already exist from create_dm.
+    let existing = conv_store
+        .load_conversation(&conv_id)
+        .map_err(|e| SdkError::Storage(e.to_string()))?;
+
+    let conv = if let Some(mut ex) = existing {
+        ex.mls_group_blob = mls_blob;
+        ex.member_keys = member_keys;
+        ex.is_hybrid = member.is_hybrid();
+        ex.last_activity_ms = now_ms();
+        ex
+    } else {
+        Conversation {
+            id: conv_id.clone(),
+            kind: ConversationKind::Group {
+                name: format!("group-{}", conv_id.hex()),
+            },
+            display_name: format!("group-{}", &conv_id.hex()[..8]),
+            mls_group_blob: mls_blob,
+            keystore_blob: None,
+            member_keys,
+            unread_count: 0,
+            last_activity_ms: now_ms(),
+            created_at_ms: now_ms(),
+            is_hybrid: member.is_hybrid(),
+            last_seen_seq: 0,
+        }
+    };
+
+    conv_store
+        .save_conversation(&conv)
+        .map_err(|e| SdkError::Storage(e.to_string()))?;
+
+    debug!(conv = %conv_id.hex(), "joined group from Welcome");
+    Ok(conv_id)
+}
+
+// ── MLS state persistence ───────────────────────────────────────────────────
+
+/// Save MLS group state into a conversation record.
+pub fn save_mls_state(
+    conv_store: &ConversationStore,
+    conv_id: &ConversationId,
+    member: &GroupMember,
+) -> Result<(), SdkError> {
+    let mut conv = conv_store
+        .load_conversation(conv_id)
+        .map_err(|e| SdkError::Storage(e.to_string()))?
+        .ok_or_else(|| SdkError::ConversationNotFound(conv_id.hex()))?;
+
+    conv.mls_group_blob = member
+        .group_ref()
+        .map(bincode::serialize)
+        .transpose()
+        .map_err(|e| SdkError::Storage(format!("serialize MLS group: {e}")))?;
+    conv.member_keys = member.member_identities();
+    conv.is_hybrid = member.is_hybrid();
+
+    conv_store
+        .save_conversation(&conv)
+        .map_err(|e| SdkError::Storage(e.to_string()))?;
+    Ok(())
+}
+
+/// Restore a `GroupMember` from a conversation record.
+///
+/// Returns `Err` if the conversation has no MLS group blob.
+pub fn restore_mls_state(
+    conv: &Conversation,
+    identity: &Arc<IdentityKeypair>,
+) -> Result<GroupMember, SdkError> {
+    let group_blob = conv
+        .mls_group_blob
+        .as_ref()
+        .ok_or_else(|| SdkError::Crypto("no MLS group blob in conversation".into()))?;
+
+    let mls_group = bincode::deserialize(group_blob)
+        .map_err(|e| SdkError::Crypto(format!("deserialize MLS group: {e}")))?;
+
+    let ks = quicproquo_core::DiskKeyStore::ephemeral();
+    let member = GroupMember::new_with_state(Arc::clone(identity), ks, Some(mls_group), conv.is_hybrid);
+
+    Ok(member)
+}
+
+// ── Helpers ─────────────────────────────────────────────────────────────────
+
+/// Optionally wrap data in hybrid encryption.
+///
+/// If `is_hybrid_mls` is true, MLS already provides PQ protection and we skip
+/// the outer envelope.
+fn wrap_hybrid(
+    my_kp: Option<&HybridKeypair>,
+    peer_pk: Option<&HybridPublicKey>,
+    data: &[u8],
+    is_hybrid_mls: bool,
+) -> Result<Vec<u8>, SdkError> {
+    if is_hybrid_mls {
+        return Ok(data.to_vec());
+    }
+    match (my_kp, peer_pk) {
+        (Some(_), Some(pk)) => {
+            hybrid_encrypt(pk, data, b"", b"")
+                .map_err(|e| SdkError::Crypto(format!("hybrid encrypt: {e}")))
+        }
+        _ => Ok(data.to_vec()),
+    }
+}
+
+/// Enqueue a payload to a peer via the ENQUEUE RPC.
+async fn enqueue_to_peer(
+    rpc: &RpcClient,
+    recipient_key: &[u8],
+    payload: &[u8],
+) -> Result<(), SdkError> {
+    let req = EnqueueRequest {
+        recipient_key: recipient_key.to_vec(),
+        payload: payload.to_vec(),
+        channel_id: Vec::new(),
+        ttl_secs: 0,
+    };
+    let resp_bytes = rpc
+        .call(method_ids::ENQUEUE, Bytes::from(req.encode_to_vec()))
+        .await?;
+    let _resp = EnqueueResponse::decode(resp_bytes)
+        .map_err(|e| SdkError::Crypto(format!("decode EnqueueResponse: {e}")))?;
+    Ok(())
+}
diff --git a/crates/quicproquo-sdk/src/lib.rs b/crates/quicproquo-sdk/src/lib.rs
index 3c82d45..38bb1c7 100644
--- a/crates/quicproquo-sdk/src/lib.rs
+++ b/crates/quicproquo-sdk/src/lib.rs
@@ -10,7 +10,9 @@ pub mod conversation;
 pub mod devices;
 pub mod error;
 pub mod events;
+pub mod groups;
 pub mod keys;
 pub mod messaging;
+pub mod outbox;
 pub mod state;
 pub mod users;
diff --git a/crates/quicproquo-sdk/src/outbox.rs b/crates/quicproquo-sdk/src/outbox.rs
new file mode 100644
index 0000000..da86f7e
--- /dev/null
+++ b/crates/quicproquo-sdk/src/outbox.rs
@@ -0,0 +1,76 @@
+//! Offline outbox — queue messages for deferred delivery.
+
+use bytes::Bytes;
+use prost::Message;
+use tracing::{debug, warn};
+
+use quicproquo_proto::method_ids;
+use quicproquo_proto::qpq::v1::{EnqueueRequest, EnqueueResponse};
+use quicproquo_rpc::client::RpcClient;
+
+use crate::conversation::{ConversationId, ConversationStore};
+use crate::error::SdkError;
+
+/// Queue a message for sending when connectivity is restored.
+pub fn queue_outbox(
+    conv_store: &ConversationStore,
+    conv_id: &ConversationId,
+    recipient_key: &[u8],
+    payload: &[u8],
+) -> Result<(), SdkError> {
+    conv_store
+        .enqueue_outbox(conv_id, recipient_key, payload)
+        .map_err(|e| SdkError::Storage(format!("enqueue outbox: {e}")))
+}
+
+/// Process all pending outbox entries — send them to the server.
+///
+/// Returns the number of entries successfully sent.
+pub async fn flush_outbox(
+    rpc: &RpcClient,
+    conv_store: &ConversationStore,
+) -> Result<usize, SdkError> {
+    let entries = conv_store
+        .load_pending_outbox()
+        .map_err(|e| SdkError::Storage(format!("load outbox: {e}")))?;
+
+    let mut sent = 0usize;
+    for entry in &entries {
+        let req = EnqueueRequest {
+            recipient_key: entry.recipient_key.clone(),
+            payload: entry.payload.clone(),
+            channel_id: Vec::new(),
+            ttl_secs: 0,
+        };
+        match rpc
+            .call(method_ids::ENQUEUE, Bytes::from(req.encode_to_vec()))
+            .await
+        {
+            Ok(resp_bytes) => {
+                if let Err(e) = EnqueueResponse::decode(resp_bytes) {
+                    warn!(outbox_id = entry.id, "decode enqueue response: {e}");
+                }
+                conv_store
+                    .mark_outbox_sent(entry.id)
+                    .map_err(|e| SdkError::Storage(format!("mark_outbox_sent: {e}")))?;
+                sent += 1;
+                debug!(outbox_id = entry.id, "outbox entry sent");
+            }
+            Err(e) => {
+                warn!(outbox_id = entry.id, "outbox send failed: {e}");
+                conv_store
+                    .mark_outbox_failed(entry.id, entry.retry_count + 1)
+                    .map_err(|e| SdkError::Storage(format!("mark_outbox_failed: {e}")))?;
+            }
+        }
+    }
+
+    Ok(sent)
+}
+
+/// Get the number of pending outbox entries.
+pub fn outbox_count(conv_store: &ConversationStore) -> Result<usize, SdkError> {
+    conv_store
+        .count_pending_outbox()
+        .map_err(|e| SdkError::Storage(format!("count outbox: {e}")))
+}