Manage iMessage group membership through space.group: 
// List current participants
const users = await space.group.participants();

// Add or remove by handle (E.164 phone or email)
await space.group.add("+15559876543");
await space.group.remove("+15551112222");
Operations apply only after the conversation is already a group. Creating a new group chat requires cloud or dedicated iMessage lines with group support enabled.

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Participant resolution

Inbound group messages identify the speaker through message.group.participant. Use that handle when you need per-member routing inside a shared group thread.

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Platform support

PlatformGroup membership
iMessage (cloud / dedicated)✅
WhatsApp Business❌ (1:1 only)
Terminal❌