Groups are a tough nut to crack. At present, a group moderator can ban users, and they can control membership by making the group private and having to approve every member, but what I'd really love to see in !GNUsocial is the ability to do Usenet-style moderated groups, where membership is open and public, but posts to the group are queued until they've been approved by a mod. That's something more like a curated content channel, and I think that's probably the style of group that most users are interested in.