I am yet to play around much with 2.1 but I understand that the Groups functionality is hierarchical in the sense that a parent can have multiple children but each child can only have one parent.

In Australian politics, we have Federal, State and Local government elections. Consequently a Group of volunteers may be a member of Federal, State AND Local campaign teams (Groups). This Group of volunteers should therefore be a child of 3 parents, or perhaps more correctly, have a relationship with 3 other Groups. It seems to me that ideally the Groups should really have a relationship to each other (other than parent/child) and that on creating the relationship type you would set permissions as to how the members and administrators of each group could interact with members of the other. I haven't thought through how that interface might work or all the intricacies, but thought I should throw the concept out there in the context of this discussion as it would seem to fit in with what lobo is proposing at bullet 2 above.