DQL many to many and count

It's not easy without seeing the actual classes, but by guessing you have a many-to-many bidirectional relationship:

$dql = "SELECT g.id, count(u.id) as cnt FROM Entity\Group g " .
    "JOIN g.users u GROUP BY g.id ORDER BY cnt DESC LIMIT 10;";
$query = $em->createQuery($dql);
$popularGroups = $query->getArrayResult();

UPDATE:

You don't have to use a bidirectional relationship, you can query the other way around:

$dql = "SELECT g.id, count(u.id) as cnt FROM Entity\User u " .
    "JOIN u.groups g GROUP BY g.id ORDER BY cnt DESC LIMIT 10;";