Encrypted group chat on nostr is being solved three different ways.
We compared Marmot (MLS for small high-stakes groups, no privileged server), Concord (Discord-style communities with shared-key streams and signed rosters, five established clients), and Cordn (MLS through a lightweight self-hostable coordinator that is blind to content, identities, and IPs, with nothing identifiable as Cordn messaging persisting on public relays).
Full breakdown with a comparison table, what the coordinator can and cannot see, and which to pick:
https://nostrdev.com/blog/nostr-group-chat-protocols