In the login user server behavior there is a tab for "Store Values" which allows you to store as many of the logged in user's fields into the session as you need. If you store their UserGroup field into the session then you can compare it and redirect on subsequent pages to restrict access to users that have a specific UserGroup value as identified by that session variable value.
For example if you stored the UserGroup value in the session as UserGroup then this code:
<?php
if ($_SESSION['UserGoup'] != "1" && $_SESSION['UserGroup'] != "2") {
header("location: login.php");
die();
}
?>
would keep you off the page and redirect you to login.php if your user group wasn't 1 or 3.