You don't. You retrieve the ftp user's password and login as that user. Otherwise you are going to run into ownership/permission problems. Your "main ftp user" is not a "super ftp user"; each user owns the files and directories it creates and gets to decide who else can access them, and no user can override that except for DreamHost staff of course.
That would be a bad idea, if you have /home/domain as a Web Directory for a domain. What you would want to do, is setup the FTP user with
and then make a directory inside it for his web files:
and then remap sub-dir on the domain
http://domain/web -> /home/username/web
The FTP user is a user on the system, and thus there will be files in his home directory that should not be accessible by the web. So this is why you create a subdirectory for his web files and tell the web site to look for files in the subdirectory instead.
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