Managing multiple domains/sub-domains on DreamHost couldn't be easier.
Basically, you just add them in the panel under Domains -> Manage Domains -> Add New Domain / Sub-Domain. Make sure you add them as 'Fully Hosted' and you will see a new sub-directory in your home directory corresponding to the new domain, it will have the same name as the domain. This is where you place your site files for this domain.
While you can do this, DreamHost is probably not the best choice if you intend to resell your hosting.
Each (sub)Domain can be attached to a different machine user. When this user logs in via FTP/SSH they will only see the sub-directories corresponding to the domains assigned to them.
If you wish to give the users access to the Web Admin Panel, you will need to create new WEBIDs (panel log-ins) for them and configure the privileges available to them.
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