Users Online?

Hi everyone, I have a login system that allows a user to login to my control panel, is there an easy way so that I can tell how many users / what users are logged into my system? What would I need to do to add this with the minimum of hassle? Would I just have to look at the sessions that are currently active and if so, how? I really want to add this feature as it will help with creating a messaging system. It might be simple but if you've never done it before it's the hardest thing in the world :-) Thank you Chris