This question has been answered by Everettg_99. See the first response.
$user = $modx->getUser();
$class_key = $user->get('class_key');
// or...
$class_key = $modx->user->get('class_key');
// or... in general for any user, e.g. user 123
$user = $modx->getObject('modUser', 123);
$class_key = $user->get('class_key');