I don’t think there’s a subdomain panel per-se as this isn’t a reseller host.
You can setup another user though, make the site host on that user and then set the permissions for that user to only edit their sites.
That way they have their own ftp login and domain management (through https://panel.dreamhost.com) without being able to modify sites other than their own.
The down side is that you can no longer access the new user’s site via FTP if you wanted, you’d have to log in with their username and password to access it.
As well i don’t think they can setup an SQL database, you’d have to set it up. This shouldn’t be a major hitch though as any site can run on one database and just use software that has the ability to user table prefixes.