Are you using chroot?
I think you want something like this
#!/bin/sh
[ "$1" = "-c" ] && a="$*"
sudo /usr/sbin/chroot /home/$USER /bin/su - $USER $a
see this link
http://tjw.org/chroot-login-HOWTO/
probably what you would do is for each real user, make a user that has a link to the same home directory of the real user and change their shell to the script above