In the Account Control Panel, add FTP or shell system users by choosing "Users > Manage Users".
Direct the people using these system users to use http://webftp.dreamhost.com/
What do you mean by main account? Each WebId can have multiple accounts in order to organize billing for services.
You probably meant system user. Obviously you have to have at least one system user, so call it your "initial" user. You can have more than one system user. By the way, there is no super system user. If you need to manage files for a particular system user, log in as that user.
Each cluster has its own system user space. Each system user has its own home directory for file storage.
Files in a home directory are accessed using FTP or SSH2 protocols. DreamHost does have an HTTP interface to FTP at http://webftp.dreamhost.com/ for use with web browsers.
Regarding web sites:
The web server must know where to find files associated with a web site. So when you add a subdoman or domain you must specify in which system user home directory the files are to be found. Usually this will be a subdirectory named after the hostname, eg /home/user/example.com. Obviously a single system user can store files associated with multiple web sites that way.
However for multiple users to store files for a single web site is not so obvious or easy.
A URL has a path just like a file or directory has a path. One can map a sub-path of a URL to that of a different subdirectory using the Domains -> Remap Sub-dir option in the Account Control Panel. For example http://example.com/second_user/ could be mapped to the directory /home/second_user/example.com
However if that doesn't work for you, then one can either share the same system user among different people, or install a CGI application that offers shared file management. And there is also WebDAV.
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