A domain, http://www.YOURDOMAIN.com, can only have one owner (FTP user). Here's a basic layout of the directories:
Those other users don't have websites associated with them unless you create subdomains owned by them, in which case, they'll have directories called SUB1.DOMAIN.com and SUB2.DOMAIN.com
In your case, you have two choices: either create a new subdomain for each user, as in the above example, or use the remap subdirectories option:
In either case, it's difficult to fully share a directory between two users, which is why both of the above examples give your new users their own place to put files.