Our company runs a web site (oursite.com) with affiliate partners who send us traffic. In some cases, we set up our affiliates with their own subdomain (affiliate.oursite.com),
In the specific case of iframes, Google doesn't say much. I've been in the same situation but I'm glad I figured it out. I posted a walkthrough here. It's in French but you won't need to speak the language to copy / paste the code. Plus there's a demo file you can download.