bbp_update_forum_group_ids( type $forum_id,  $group_ids = array() )

function bbp_update_forum_group_ids( $forum_id = 0, $group_ids = array() ) {
	$forum_id = bbp_get_forum_id( $forum_id );

	// Trim out any empties
	$group_ids = array_filter( $group_ids );

	// Get the forums
	return update_post_meta( $forum_id, '_bbp_group_ids', $group_ids );
}