bp_reg_assign_users

/**
 * Assign users to groups upon registration
 */
function group_user_activate( $user_id ) {

	if ( ! function_exists( 'groups_join_group' ) ) {
		return;
	}

	if ( is_page( 'registration' ) || is_page( 'other-registration' ) ) {
		$group_ids = array(
			13,
			7
		);
	} else {
		$group_ids = array( 13 );
	}

	foreach ( $group_ids as $group_id ) {
		groups_join_group( $group_id, $user_id );
	}

}

add_action( 'bp_core_activated_user', 'group_user_activate', 1 );