BP Group Hierarchy plugin by ddean allows group creators to place a new group under an existing group. There is currently no limit to the depth of the group hierarchy.
Every group and subgroup is a normal BuddyPress group and can have members and a forum, use group extensions, etc.
The author says: “this is an initial release, it is NOT recommended for production sites.”
Download and info: http://wordpress.org/extend/plugins/bp-group-hierarchy/