One User Per Domain Policy

Here at DreamHost we setup Domains under User Accounts as pictured below to give you a visual how this is laid out.

This allows one User Account to access multiple Domains/Websites which is beneficial for resellers, Web Developers that manage multiple domains, and our unlimited hosted domains setup. So when creating a user name, remember that this one user name can be used to access multiple websites and can be used by multiple people so you may not want to be too user specific when naming the user.

Example: Lets say you will be creating a User for your companies website, and there will be multiple people using this FTP user login.
You wouldn't want to name the User Account after you 

Bob_Smith

instead go for a name that is more domain/company-centric like 

Domainname_Admin

Some other Hosting providers setup their User Accounts to be under Domains which allows multiple User Accounts to gain access to one Domain but makes managing multiple Domains/Websites more of a pain when you have a lot of domains.

Please note that you are not able to setup your account here at DreamHost to have this layout

If you Created a second User Account it would have its own home directory that is not connected to any of your other user accounts so you would not be able to access any Domain that is currently setup on another User Account.

We do provide an option for VPS and dedicated server customers to allow other User Accounts on your plan to access folders within a Domain that is within another of your User Accounts, this is done through Remapping a directory to be a user's Home directory as described on this page. You can see what this looks like below.

What our system does is takes the Folder in the Domain you specify and creates a "Symlink" to the Home Directory of the User you Specify. The only downside to this is that the Domain User will not be able to access the folder via FTP as it is in reality in the second User Accounts Home Directory.

You can setup Directory Re-Map from the {Domains -> Remap Sub-Dir} page of the DreamHost panel.