Multiple registration pages for different usergroups?

[Bill from Cleve]

Does anyone know how to setup or knows of a hack that will allow me to have mutable registration pages that I can assign to links directed back to my site?
I have a few dedicated links posted on different types of websites. I want to see if there is a way to have a different registration page made for each site and will assign a different usergroup based on each link.. Basically multiple default usergroups assigned to multiple registration links "register.php defaults to membergroup and register2.php defaults to membergroup2.php”
The problem I have is many different types of people come to my site. For the people that like something like heavy metal and are coming from a link posted on a heavy metal website, it will direct them to lets say "register3.php" that has "membergroup3" already assigned as the default.. So let’s say Mary poppins from site another linked site registers, she gets a nice friendly theme and doesn’t get a SLAYER theme.
I was looking around and didn't find any solutions yet. Any help would be greatly appreciated. I will also be willing to pay. So PM me please if you are selling such a service.