Group Membership is basically being able to add yourself to an additional usergroup. Which usually allows you to gain extra features, such as a greater PM total, etc.
We can even assign group leaders to moderate the group
But I don't know how you'd incorporate it into your idea. One idea though, is having member reach for a 'higher' usergroup, where they get extra permissions, but then that leads to the argument of the forum not being equal.
Alcar...