Found this: How do I view login history for my PC using Windows 7
The best way is to create a logon/logoff script that is run when each user does either. It can record the logon/logoff info in a file somewhere.
The logon/logoff info is recorded in Event Viewer but it would be cumbersome at best to view it that way. I'd recommend downloading PSTools
and using PSLogList to extract the records you want, write them to a file and process them to generate a chart or whatever output you need. Edit: psloglist security
will dump the security log which is where logon and logoff info is stored.