I just stumbled onto this post and thought I'd mention that the MODX user getList processor will filter by usergroup.
$fields = array(
'limit' => 0;
'usergroup' => $groupId;
);
$users = $modx->runProcessor('security/user/getlist', $fields);
The result will be a JSON object and you'll get some fields you may not want. The getList processor takes a 'where' property, but unfortunately, it can only be used to search for a specific user.
