Google Directory API - batch add members to a group - google-admin-sdk

Question

I am using the Google Admin SDK to create, update and delete mailing lists (aka groups).
Everything works fine so far and I can create a new group and add members to it. But: Every adding of a member takes about 1s so I was thinking of a batch request to add several users to a group at once.
In the Google Admin interface it is easy to add several users at once but I didn't find any way to implement this via the API.
Is there a way to do so or do I have to loop through every user?
This works but takes a lot of time if I have to do it for every single user:
$service = new Google_Service_Directory($this->getGoogleClient());
$user = new Google_Service_Directory_Member();
$user->setEmail('test#test.com');
$user->setRole('MEMBER');
$user->setType('USER');
$service->members->insert($group_id, $user);

Answer 1

finally I found a solution on my own: The Admin SDK comes with a Batch class :)
To get batch requests working these steps are necessary:
When initiating the Google Client add the following line to the code
$client->setUseBatch(true);
then you can initiate the batch object
$batch = new Google_Http_Batch($client);
a little modification on the code posted above brings me to this code
foreach($arr_users as $user)
{
$userdata = new Google_Service_Directory_Member();
$userdata->setEmail($user);
$userdata->setRole('MEMBER');
$userdata->setType('USER');
$batch->add($service->members->insert($temp_list_name, $userdata));
}
finally you have to execute the request which is done by this line:
$client->execute($batch);
that's all and it works perfectly

Answer 2

While using the method of Christian Lange I was getting this error -
Argument 1 passed to Google\Client::execute() must implement interface Psr\Http\Message\RequestInterface, instance of Google\Http\Batch given,
So I used this instead
$client->setUseBatch(true);
$service = new Google_Service_Directory($client);
$batch = $service->createBatch();
foreach ($emails as $email)
{
$user = new Google_Service_Directory_Member(array('email' => $email,
'kind' => 'member',
'role' => 'MEMBER',
'type' => 'USER'));
$list = $service->members->insert($key, $user);
$batch->add($list);
}
$resultsBatch = $batch->execute();