Changing the shoppers group by api calls?

[j.power40]

Hi i would like to change the shoppers group when they submit a form
1: they fill out a form
2: on the form submit it grabs their user name and calls a php script
3: the php script changes the the virtuemart shoppers group to 'TaxExempt'

i need to know how to call this functionality from a php script please?