I suppose it is "reasonably" secured (though you have to have complete faith in your "users'" abilities and actions), and , while it may not be "impossible" it would be very complicated to set up and maintain (you'd have to become very familiar with the tools I mentioned in my previous post). An example of what I was talking about can be found in the DH Wiki article on Unix Groups, and some more info in the DH Wiki article on Unix File Permissions (and the "Cookbook" article linked at the bottom of that page).
Another approach is to just build your directory structure as you want, but retain all the ownership of the files yourself while allowing users to upload into various directories via a script, instead of via ftp as "full-on" *nix users.
To do this just upload/configure an appropriate script in each directory you want the users to be able to upload to and let them use the script to accomplish their uploads.
I have used ffileman for this purpose, and it works very well. It is easy to install, allows you to control what types of files are uploaded, and presents an interface to the user that is simple and functional. You can have multiple users "share" a directory by allowing them to share a user/pass combination, and you will be set.
The end result should be the same (users can upload to, and manipulate files in, only certain directories) but your life will be much easier, as *you will "own" all the files.