To clairfy in addition to what Scott says....
No, it is not possible on DreamHost. You don't get to control how the FTP program is ocnfigured. All FTP users have their own "home" directory where they get to store their own files. The home directory is not meant to, and should not be, web-accessible. If the user is using FTP, the FTP server keeps you inside the home directory (ie / = /home/username) but if the user uses SCP/SFTP/SSH they are not restricted to the home directory.
When visting a web site, the url-path of the URL will be checked if there is an alias to a directory on the filesystem. If there is no alias it is mapped to the document root, or "web directory" as DreamHost calls it in the Manage Domains panel. In the DreamHost Web Panel, the alias is called "Remap Sub-dir" instead of alias.
Thus you can alias a url-path to a sub directory of a different user than that specified for the web directory as Scott mentions. It doesn't allow for sharing management of the files though.