<?php
$data = [
'access_token' => 'ключ_доступа_сообщества',
'v' => 5.95
];
$url = 'https://api.vk.com/method/messages.getConversations?' . http_build_query($data);
$data = file_get_contents($url);
$json = json_decode($data, true);
$users = [];
foreach ($json['response']['items'] as $item) {
$userId = $item['conversation']['peer']['id'];
$users[] = $userId;
}
print_r($users, true);