BuddyPress.org

Read this again:
https://buddypress.org/community/groups/how-to-and-troubleshooting/forum/topic/bbpress-setup-through-buddypress-plugin/?topic_page=2&num=15#post-51853

You can’t create sub-forums for a group natively in BP. It’s potentially possible to create a plugin to create sub-forums for each group, but such a plugin doesn’t exist right now.

If you need sub-forums, stick with the external install of bbPress.