The software that actually runs the web site is call an web server.
The web server is configured to look for files in a directory called the document root. When activating a domain or subdomain in the DreamHost Web Panel, the document root is called the "Web Directory." This document root is the path to a directory in the home directory of a single user.
However one can configure the web server to map URL paths to a directory outside the scope of the document root. One configures the web server to do this using the Alias directive. However the DreamHost Web Panel has to do it for you and its called "Remap Sub-Dir".
For example, if your domain is example.com, and you want http://example.com/second_user/ to show files uploaded by second_user, then you use the Remap Sub-Dir panel to map the URL path /second_user/ to the a subdirectory /home/second_user/web. It's generally not a good idea to map a URL path to a home directory itself.
If you don't intend to map URL paths to directories belonging to different users, and actually intend for more than one person to manage the files for the domain, just share the username and password of the user managing that domain.
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