Update Usergroup using Php Script if email already exist

[warmarks]

Hi,

I am trying to update usergroup of a given user whose email is known in $email variable.

PHP Code:

<?php 
define('VB_AREA', 'External');  
define('SKIP_SESSIONCREATE', 1);  
define('SKIP_USERINFO', 1);  
define('CWD', '/home--path');  
require_once(CWD . '/includes/init.php');  
// Data from Form Submision for VB 


$email  = $_GET["email"]; 
$usrgrp = $_GET["usergroupid"];

Can you please guide me what code i need to add in there...
so that if a user with $email is member of any usergroup before, i can add him to usergroup 3
also when $usrgrp=3,

i mean i know the email of the useraccount, and want to add him to additional usergroup (not removing from the existing ursergroups) i am using above php code from a non-vb page.

Looking forward to receive some help in this regard.

Thank you!