Assuming that you know about and are not interested in anonymous ftp, I have not found a way to do that using standard ftp clients, as Dreamhost allows only one "user" to own an ftp directory.
From the old "knowledge base", current wiki.dreamhost.com entry:
That said, you might consider a php/perl filemanager or groupware application that will let you set up a "repository" of documents/files where multiple users can upload/download common documents. As the cgi user "owns" all files, you will be relying upon the application for access control and security, but your user (or which ever user you designate in the panel for the domain to run cgi as) owns then all.
I think this is a better solution than trying to kludge together a series of unix users, groups, directories, and symlinks (which I suppose you could cobble together after a fashion).