[Opserty]

1. Create the custom fields using the AdminCP and make sure they are required & displayed at registration
2. Create some custom PHP code in one of the registration Hook Locations I'm thinking 'register_addmember_process'. With some PHP code like:

PHP Code:

if(...)
{
    // X is a positive interger (the ID number of the usergroup)
    $userdata->set('usergroupid', X);
} 


Hopefully something like that should work, it will need a bit of testing but it shouldn't be too difficult. If you know PHP you could probably take a bash at it yourself, otheriwse you might need to hire someone to do it for you.