Multi-URL Website Setup/Architecture

Lets say you want to set up a forum site at mainforum.com.

Then you want to add a subdomain where the URL xyz.com would point to something like xyz.mainforum.com. All of xyz.com's forum questions will show in mainforum.com, but if I go to xyz.com, only the questions originated there will show.

This is kind of like a multi-tenant type app, but I want one user to be the tenant of all the subdomains and main page. The forums from xyz.com would show up in mainforum.com and even reference the URL where they originated.

Each subdomain would be customizable with things like a header image, logo, about us page, contact us page, and even send emails from that domain.

I hope that makes sense. So basically 1 admin for a main site and multiple sub-domain sites where all the "child" sites have a unique URL, but share their data with the main site. The users could have one set of credentials for all the sites.

Anyway, and suggestions on how to lay out the architecture for something like that would be appreciated.