Ok, before the revamp, I could add a user to a forum group (if I owned the group or am admin, I think?). I can't find this feature anywhere, even when I changed the group joining mode to "invite only".

Anyone have any ideas? Or should this go in problems & bugs forum? OTL

this is a quote from in a locked thread:

http://subeta.net/forums.php/read/818743/Forum-Group-Updates

Someone asked if they bringing back invites, and they told its being worked on. That was 11/5/14.

Yeah, this is a known bug, has been around since the new groups were implemented :/ Hopefully they'll have time to get around to fixing it after Luminaire x_x