You could use login and logout macros assigned to every user. When any user logs in run a macro that sets a bit in the PLC, and when any user logs out run a macro that clears the same bit.
Of course you can get more complex if you wanted, but if your only objective is "is somebody logged in or not", then that will do the trick.
Edit: just saw you're using the Panelview 5000 series, which I've not used, so my advice may not be applicable. Do the PV5000's have login/logout macros?