Check Which User Has Window Open

i installed a shout box for my users, its integrated by using session username. but what i want to do is have a username list under the shoutbox that shows which members (usernames) have the box open. that way the other members can see someone is out there to see their message.. one alternative would be to just have the shoutbox auto open (which i need to learn how to do) as soon as they sign on to the site, but what if they close it right away i still have the same issue (they cant see the message that someone is sending them) here is how i open the window now, they have to click on the link.

<a href="#" onClick="javascript:popup=window.open('shoutbox/minichat.php','SHTB','status=0,menubar=0,resizable=0,scrollbars=0,width=297px,height=300px');"><font size="2">Shoutbox</font></a>

so how do i know which users have it open? and how to i autostart the shoutbox? (do i used onload and onunload?)