You don't add users to domains.
The web server is configured to look for the files in a subdirectory of a particular user. This is known as "DocumentRoot" in the Apache documentation, and "Web Directory" when setting up a fully hosted domain on DreamHost.
Besides DocumentRoot, Apache can also use "Alias" to map a url-path to a path different than that of DocumentRoot, allowing you to add "subdirectories" that other users can upload files for.
This is known as "Remap Sub-dir" on DreamHost.
If what you are actually requesting is the ability for two people to upload files and the files, regardless of who uploaded them, visible from the same url-path, then you options are:
- create single FTP/shell user and the people share the username/password (easiest)
- setup a group and use group permissions etc.
- Possibly use WebDAV if it meets your needs
Perl / MySQL / HTML+CSS