what kind on an info do you want to keep?

Basically, the Ip address can be the identifier column for your table and you can keep any user that his IP does not exists on the database.

to that very basic 1 column table you will add to each row any other information on the user that you want (you can look for the info on the serverVaraibles list).

more advanced, to each row you can add the current date and time and get more specific on your statics (e.g. alltime, last year, last month, last week, today, 5 minutes ago - online).

i mean to something like this: http://extremetracking.com/open;unique?login=alexucho

if you get troubles i can help.