How do I allow users to have access to only a specific subset of folders on my Ubuntu server?

view full story

http://serverfault.com – I need freelancers to be able to download and upload files to some of my websites on my Ubuntu server, but not all. How do I set it up so that each freelancer is a user who has access to only the sites that they should be able to access and nothing else? I've followed the instructions I found to set up ftp users with access to particular folders via bind. I have added my ftp users to a group that has read, write access permissions to those folders, so all seems good from that perspective - my users can do what they need to do. But my ftp users are able to navigate to all the folders on my s (HowTos)