public function addFriends($user_id, $friend_id)
    {
        if (!is_numeric($user_id) || 0 >= $user_id) {
            throw new InvalidArgumentException(
                "Api::addFriends user_id field should be positive integer");
        }

        if (!is_numeric($friend_id) || 0 >= $friend_id) {
            throw new InvalidArgumentException(
                "Api::addFriends friend_id field should be positive integer");
        }

        /**  @var $command \Guzzle\Service\Command\AbstractCommand */
        $command = $this->client->getCommand('AddFriends', array('id' => $user_id, 'user_id' => $friend_id));

        return $this->executeCommand($command);
    }