If you're using the desktop version of OS X (not the server version) then you're pretty much stuck with the default settings for the built-in FTP server. That means that FTP users see the home directories no matter what, and the FTP username and password is linked to the account. So if you want another FTP user, say "bob" with password "ftpmenow", then you create a normal OS X user with username "Bob Smith (or whatever)" and shortname "bob" with password "ftpmenow".
Mac OS X Server does not have these limitations, and I find the FTP server to be quite robust. I also don't know if Mac OS X Desktop supports SFTP, but Server does.