dee-u: it is already possibly to subscribe to a discussion just like a regular forum thread. However, and kind of notifications for new discussions would be valuable.

Personally I think the social groups feature of vBulletin simply does not work well. It is invisible in so many ways and it is cluttered in the way it is designed. In the other hand I haven't joined any social groups so I may be missing some features related to it. If you join a social group does it appear in the front page? That would make it a lot better.