BuddyPress.org

That “section” could be a “group” component already in WordPress and joining groups is either voluntary for public groups or by invitation for private groups and each group can have a forum if the group admin so chooses. That’s basic behavior which can be changed via a plugin you can make to do so. You could check out BuddyPress Group component and other default components by logging in http://testbp.org/ with the username and password you used here.