One community I am building has interest groups within interest groups…e.g. fine arts > bodypainting > henna
Another one is in an academic setting and for study groups..there are class-level activity and then there is individual subjects within each class for which there are study groups.
So for us, it would greatly increase the utility of BuddyPress if there were hierarchical groups. And a member in a lower level group automatically becomes a member of a higher level group.
Thanks!