How to identify who are online?

[eluser]InsiteFX[/eluser]
You could use a cookie.

When user login, create a cookie with there information for your table.
Make sure the cookie last for a week or so.

Now when the user logs back into your system you can check for the cookie
update your table, then delete and recreate a new cookie with the new
information.

I am sure there is a better way.

InsiteFX