BuddyPress.org

I reread your post and deleted what I had previously written. If I understand your request, wouldn’t hidden groups be the solution? You can already do that in BuddyPress.

If Group A and Group B are both hidden groups, then only members within a given group would have access to the activity stream of that group. So, Group A members wouild not be able to see Group B activities and vice versa. The ony exception to that would be if a user was a member of both groups.