Personally I suspect it is a different problem. As when I tried to join the group I got the invite almost immediately. It's possible Mobius was online and did respond that quickly but it made me wonder. Unless the group can be set to open and then invites still have to be sent out?
I wonder if perhaps the invite to join is a lie, and the request to join hasn't been approved yet, and thus why you can't actually join the group instance. That the gave showing you that you received an invite is the actual bug, the it shows that prematurely before the group owner has had a chance to respond to your request to join.
