public static function get_inviter_ids( $user_id = 0, $group_id = 0 ) {
		global $wpdb;

		if ( empty( $group_id ) || empty( $user_id ) ) {
			return array();
		}

		return groups_get_invites( array(
			'user_id'     => $user_id,
			'item_id'     => $group_id,
			'invite_sent' => 'sent',
			'fields'      => 'inviter_ids'
		) );
	}