function groups_leave_group( $group_id, $user_id = 0 ) {

	if ( empty( $user_id ) )
		$user_id = bp_loggedin_user_id();

	// Don't let single admins leave the group.
	if ( count( groups_get_group_admins( $group_id ) ) < 2 ) {
		if ( groups_is_user_admin( $user_id, $group_id ) ) {
			bp_core_add_message( __( 'As the only admin, you cannot leave the group.', 'buddypress' ), 'error' );
			return false;
		}
	}

	if ( ! BP_Groups_Member::delete( $user_id, $group_id ) ) {
		return false;
	}

	bp_core_add_message( __( 'You successfully left the group.', 'buddypress' ) );

	/**
	 * Fires after a user leaves a group.
	 *
	 * @since 1.0.0
	 *
	 * @param int $group_id ID of the group.
	 * @param int $user_id  ID of the user leaving the group.
	 */
	do_action( 'groups_leave_group', $group_id, $user_id );

	return true;
}