I don't believe there is an "out of the box' way to do this. One way of doing it would be to create a workflow that moved the user into the correct group before it was published.
You must make an new workflow and add an new workflow event, set it to run on after publish. Basically the workflow event (which you must make) must.
Fetch the object currently published
Check the class id, if class id is user class id then
Check the user attribute which controlls to which user group the user go to Then move the user object to the correct user group
See http://ez.no/products/ez_publish_cms/documentation/building_an_ez_publish_site/the_guestbook/creating_a_workflow on how to add an workflow. I advise you to check out other customized workflows on how to make one yourself.