<?php
use Microsoft\Graph\GraphServiceClient;
use Microsoft\Graph\Generated\Groups\Item\Members\Graph\User\Count\CountRequestBuilderGetRequestConfiguration;
$graphServiceClient = new GraphServiceClient($tokenRequestContext, $scopes);
$requestConfiguration = new CountRequestBuilderGetRequestConfiguration();
$headers = [
'ConsistencyLevel' => 'eventual',
];
$requestConfiguration->headers = $headers;
$graphServiceClient->groups()->byGroupId('group-id')->members()->graphUser()->count()->get($requestConfiguration)->wait();