BuddyPress.org

Howdy. You can access the bp group forums from Groups > My Groups > <pick a group> > Forums. The bbpress theme doesn’t get changed by doing user integration with wpmu.

It sounds like you have one more step to take and that is to configure bp group forums. That is detailed in the file /mu-plugins/bp-forums/installation-readme.txt

There is also an extremely helpful topic by Trent located here: https://buddypress.org/forums/topic.php?id=471