I need a few clues on how best to go about this:

1. I want to build a site where my user is required to open an account and login.
2. Upon login they're presented with a link containing their affiliate version of the site. Basically something like mydomain.com/theirID.
3. I would then need their specific ID passed on to other pages of the site.

So basically it's just a replicating site like a lot of vendors have, but I'm just not sure how to go about that. Any tips?