Have you checked out managing "accounts and privileges". In there you should be able to set user names and passwords. I do not knwo about FM8, but in 10 + you can make the user reset their password on next login. You may be able to usea script to trigger a password reset after each loggin.
Does this get at what you need?
I'm not sure that accomplishes what I'm after. I want to prevent two simultaneous users on the same account and any one time, but I don't want it to be a hassle for them either... resetting passwords constantly, etc.
Thanks for the comment.
You could try using a scrip set to run when the file opens to log the user's account name. This script could first check to see if anyone else is already logged in via the same account name that hasn't logged out. You'd also need a script that runs when the file closes that logs the fact that a given account name should be logged out.
The catch here, is that if a user is disconnected--which can happen fairly easily, they won't be logged out and then the system would then deny them access until you or some administrator fixes the log entry.