Maybe this thread will help?
http://www.belltreeforums.com/showthread.php?333824-Notification-Message-not-going-away
Uhh it's been a while since I've fiddled with groups, and I remember this happening to me ages ago, and I can't remember how I fixed it. Maybe try leaving the group to see if the notification goes away, and then join on your own request without the invitation? But if it's still there after you leave them just accept it?
I have just left but it's still there
Have you tried logging out and logging back in?