You can resell your space on any plan, but the plan’s aren’t setup like a CPanel reseller account, if that’s what you’re used to. Dreamhost wouldn’t handle the billing for your customers, though–that would be up to you.
You can add multiple domains, FTP users, shell users, etc…
Here’s some info in the DH Wiki about it: http://wiki.dreamhost.com/index.php/KB_/Sign_Up/_Reselling
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