Automatically update User Group ID

[jw4]

I want to automatically put users in a user group when they have confirmed registration, depending on the value of an extra user field.

I want to put the code where the user group value gets updated from 'awaiting email confirmation (ID 3) to 'registered' (ID 2) - but I can't find which php file this is in.

I have looked in userjoin.php and login.php but can't see it - can anyone tell me where this happens?

Any help much appreciated!