Adding a forum to a group when BuddyPress and bbPress are installed is part of the group creation process,so you – or anyone with Admin level capabilities – would make the choice either when the group is set up or later as part of a review process.
bbPress works without buddypress so it is possible to add forums without installing BuddyPress.You would have to evaluate the features of each plugin that are essential or optional and then make a judgement about the combination.
The easiest way to evaluate the features is to set up a test site and play with different combinations.
