For that, people need to voluntarily log in somehow. Just like here at PerlMonks, for example. Here, we log in and accept a cookie. On future visits (assuming we've got the "keep me logged in" option set) that cookie announces our presence. Then each time we hit the server, our "last seen" timestamp gets updated. The "Other Users" nodelet here at the Monastery, which shows who else is currently logged in, is actually a "Who has hit the server with a recognized cookie in the last ten minutes".

That's about as good as you can do on the web. There is no true "logged in" state for the web; just "recently here". This isn't a Perl situation, it's a condition resulting from the fact that the Web is stateless.