public function addDeviceToGroup($senderId, $notificationKeyName, $notificationKey, array $registrationIds) {
    if (empty($notificationKey)) {
      throw new \InvalidArgumentException('notificationKey cannot be empty');
    }

    return $this->performDeviceGroupOperation($senderId, $notificationKeyName, $notificationKey, $registrationIds, self::DEVICE_GROUP_ADD);
  }