Figure out what you're willing to do, and not to do. What happens if they want more disk space? What happens if their subdomain starts using up a LOT of bandwidth? Will they be allowed to store files? How will they update the page? You aren't running a business in the sense that you're collecting money from them, but you are, in that you're exchanging services (hosting for forum modding). You'll want to think of these issues beforehand, and provide them and yourself with a clear understanding of what's expected and allowed - from all parties.

...and the wet blanket will now shut the heck up.