1) The same as in a two-user chatroom? To hide the conversation content from everyone outside of the chatroom? 2) I heard people work on that, but I don't know

1) The same as in a two-user chatroom? To hide the conversation content from everyone outside of the chatroom? 2) I heard people work on that, but I don't know any details. Might take a bunch of pairwise exchanges [tedious, but probably okay for small rooms], or moderators may serve as trust/key exchange hubs.