Until the group PMs started being usable (and they’re still not quite there), we used categories that were only viewable/postable by the specific groups. That, coupled with my version of the Watch Category plugin was a decent solution. But as Tobias mentioned it was really confusing to users.
I’m not sure that Tobias solution is ideal though. Right now it’s a vague whether something is a group PM or not. I don’t have anything to suggest at the moment. I just wanted to chime in and say that I’m facing the same difficulties as Tobias.