The first post in this thread was made 7 months ago when bbPress needed to be separately installed.
As of BP v1.1+, bbPress is now embedded into BuddyPress.
Privacy options for group forums now exist natively.
If you’re using an external bbPress install (which it sounds like you are), then you’re correct, privacy will still be an issue. I’m not even sure the BP Groups plugin by Burt Adsit even works with BP 1.1 / external bbPress.