Handling multiple domains/sites on DreamHost is about as easy as it gets.
You can administer all domains using a single panel log-in (or WEBID), or create new WEBIDs and give them various panel privileges.
The same thing applies to FTP/SSH log-ins, you can have all domains under the one FTP user, or you can have each domain under a different user.
I should note that each domain (and sub-domain) has its own web accessable directory where you place the files for that particular site.
Overall, the DreamHost system really is very versatile.
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