mirror sites? not sure if that is what I need

Thanks for reading

I am working for a client who does multi-level marketing.
I just finished his web site and now he has asked about mirrored
web sites. I am not even sure if that is the right term.

I hope I can explain it right. He has ppl sign up and they receive an id
number. When a person who is a member then sends someone to the web
site in order to get them to sign up, he want the new person to be
directed there via a link which contains the referrer's id so the new
person is signed up under the referrrer's position. Of course, the
referrer received the link when he signed up and gives it to the new
person.

I have seen this done on some site in the past, no matter where one clicks
there is always the referrer's id in the url so the referrer gets credit
for the visit if and when the new person buys or signs up for something
the site is selling.

Is this done via a script of some sort or is it a mod implemented on the
server of the web site. Its all new to me. my guess it is a combination
of the two.

Someone here must have set up something like this before, appreciate any
help.

btw, here is the web site http://www.weeklytrade.biz
just in case you wondering...

thanks
Helaman