With dreamhost, only one user has access for FTP or or shell for each domain. That user is the one you selected in Panel > Domains > Manage Domains > Edit. Let's say joe is the user you can log into ftp with and access domain1.com. Sally is the user you can log into FTP with and access domain2.com. Now, if you create the user Jim Bob, and he's not associated with any domain.
Joe can only upload to Domain1.com, Sally can only upload to Domain2.com, and Jim Bob can upload to his space, but it's not web-accessable.
You can use directory re-mapping (search the K-base on how to do this) So that Jim Bob's 'cool-stuff' folder actually goes to domain1.com/cool-stuff/.
There's no way to set it up so Jim Bob can access all of Domain1.com - only his directory (and sub-directoresi there of)
Does that make more sense?