I have sessions working and keeping track of any variables I need to keep track of but I am having troubles when it comes to figuring out how to track the number of people on my site and who they are.

Now I have made it so that anytime someone views my page a session is set and they are added to a database which tracks their session id along with other various information such as their username, but what I cannot figure out is knowing when they leave my site. I just cannot figure out a decent way of going about that.

Any ideas? Tutorials?

-Matt