The original V6 kernel and file systems used a char for UID and GID.    This meant that you could only have 255 (plus the root user) distinct users on the machine.     The JHU PDP-11/45 was used for the EE classes and we had more than that many users.    The kernel was modified to check if the GID was 200 or greater.   If it was, that was taken along with the UID to be part of the user identity.    We gave all the class accounts such GIDs.

Of course, we had to be careful about newgrp and fun and games with setuid/setgid (both the system call and the bits on the executables).

I spent a lot of my time looking for exploits there and fixing them once I (or someone else) found them.