[DragonBlade]

Well, I dunno if I'd wanna do it for pay, but I'd be willing to help YOU do it. :P

And yes, it's very possible to do. First, you've have to make sure you can allow outside sites access to your database (the sites that you want to place the registration page on in particular).

Next, you'd put a hidden form element in the registration form on each page with the usergroup ID that you want someoen to belong to.

Finally, you'd make a plugin that would assign said usergroup to an individual upon registration. :3 Pretty simple, and I'll help walk you through the steps if you're patient.