@Trublz yeah we've both tried relogging, it still doesn't work a lot of the time, but did once last night after many times trying.
Suppose that's the only fix.
Ah that sucks. Only advice I can give is to submit a ticket and just try again tomorrow. This has happened to me and others miltiple times. Generally it seems to go away eventually.
Also have you tried grouping with someone else? If you can group with anyone else, get them to invite you and your friend and then have the invitey leave group.