I understand that the shell users have access to the website, but they don't (and can't) have access to any other features of my server, the fact being that their UID is the one that will be used to serve my web-pages, and should there be a bug in my website, I can't let them have any access to any files other than the website. The single-UID paradigm leads to this problem.
Adding more people to the client's group, and making the web/ folder group rw doesn't incur any further security holes, as the webX user can still only access what it could before, yet I can also use my normally set up users to alter the website. Obviously, if the client has more websites, then the other shell users could access the other website, but this is a non-issue; worst case scenario, I can have only one website per client.
If there is no way to configure this, a pointer as to where it is in the source would be nice.