Why not set up a sub domain for each user? Then they could ftp into that directory and not have access to the rest of your site?
For example, let's say you have a user we'll call weebles.
In CPanel, set up a sub domain for weebles. When you ftp in, you would then use for the ftp address: weebles.****** The username would be: weebles@****** with the password assigned to that user in your e-mail setup. This would give them access to only the weebles folder that will be created when you create the "weebles" sub domain.
Create as many sub domains as you need.....
Just a thought.......Hope this helps