The easiest way would be to place a default timeout per session. For example:

Consider a user to be logged in for five minutes after they do something. Meaning when a user loads a page, it writes his name to the 'logged in' file. Five minutes later an external script removes him from the list. When he browses another page, it re-adds him back to the 'logged in' file. It's quick and dirty, and isn't very accurate, however it should be accurate enough for what you are wanting to do.

There are 'better' (better being quoted because what is 'better' is relative to your unique situation) ways of doing it, but the web was designed to be a connectionless medium so trying to track "connections" isn't that easy/accurate.