You add a domain or subdomain for the new user via Panel.
Domains > Manage Domains > Add New Domain / Sub-Domain
Fill in the domain.com (or [sub].domain.com) that you want to give the user.
There's a spot saying FTP user / CGI-runs-as user: with a dropdown box right next to it. Just select your user's username there.
The domain will show up as a "domain.com" folder in the user's ftp. Everything in there is the www bit
Acces via http://domain.com (or http://sub.domain.com if you've setup a subdomain)
Tip: A handy test for a 1st-try is to create a domain called [blahblah].dreamhosters.com
Use something random where the [blahblah] bit is.
Maximum Cash Discount on any plan with MAXCASH