@shoq when it comes to groups, there are two parallel implementations in @nodebb:
1. "user groups" which — true to name — is a group of users. They can be mentioned in posts and will be notified thusly.
2. Group chats, which function almost exactly like "mentioned-only" visibility in #mastodon. Users are added to the group and messages exchanged only show up to the users in said group.
Is there a third use case you'd like to see, or is it covered in the above?
