Good morning Jeremy :)
I'm just thinking on the fly about this. Would you need a running history or just currently logged in and last time logged in?
Possible solution ( current & last )
In the Users Table, I'd add at least (2) more fields.
1. Current (timestamp)
2. Last (timestamp)
And maybe a Logical On/Off field that you would Update when folks login (toggle field = '1') and update when they logout (toggle field = '0') .
Just a rough game plan.
Once you've got that all working, you' be able to do a display all users currently logged in without too much trouble.