<?php
use Microsoft\Graph\Beta\GraphServiceClient;
use Microsoft\Graph\Beta\Generated\Models\AadUserConversationMember;
$graphServiceClient = new GraphServiceClient($tokenRequestContext, $scopes);
$requestBody = new AadUserConversationMember();
$requestBody->setOdataType('#microsoft.graph.aadUserConversationMember');
$requestBody->setRoles(['guest', ]);
$additionalData = [
'user@odata.bind' => 'https://graph.microsoft.com/beta/users/8ba98gf6-7fc2-4eb2-c7f2-aef9f21fd98g',
];
$requestBody->setAdditionalData($additionalData);
$result = $graphServiceClient->chats()->byChatId('chat-id')->members()->post($requestBody)->wait();