Im wondering the same thing Salathe is...

Id simply make a table "loggedin" with coloums "username" "ip" "date"
when they login just add that there, you can make date the date and the hour or something, then just search for that username and the date your in

Example: date('d-m-Y-G')
blayne4k 127.0.0.1 04-28-08-10
blayne4k 127.0.0.2 04-28-08-10
blayne4k 127.0.0.1 04-28-08-11

select only the ones from that day and hour, and group by username, put a count on it and you can see how many people login that hour

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