If you install standalone bbPress at example.com/forums/, then you need to tell BuddyPress to change their forum slug to something else. Otherwise, there is a slug collision.
This can be done by adding the following line in wp-config.php:
define ( ‘BP_FORUMS_SLUG’, ‘discussions’ );
“discussions” can be changed to anything you desire. Now your BP group forums will reside at this new slug.