BuddyPress.org

You could put it in functions.php, but (and I am copying this from the BP codex), you should use bp-custom.php because:

(bp-custom.php) gets loaded with all the rest of bp automatically by bp during each page load. If you don’t have one already then create one in /plugins/buddypress. So we’ll pretend that we have done just that and that our three new functions are going to be loaded with the rest of bp.