Users cannot have domains. Clusters have machines and users. Domains have hostnames, and hostnames have machines.
You only mention the FTP protocol. If all you need is FTP, then all you have to do is make one FTP user, give both people the username and password, and tell them to log into the hostname server.dreamhost.com, where server is the name the account control panel shows when you made the user. They will then be able to transfer files through the home directory of the FTP user.
It just so happens that one can use your own domain as the hostname, eg ftp.example.com, instead of the server.dreamhost.com hostname.
HTTP protocol is a different story where it is backwards: a web site only has one user. Well, its more complicated than that, but then again you haven’t mentioned if they will need web access to the files.
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