it's actually really easy, but keep in mind that it will pop up for users who are logged out.
If you turn PHP on in your header, you could say:
Code:
if (!$bbuserid) {
//$bbuserid is a cookie that has the userid - hence showing that they're logged in. If it doesn't exist, they aren't.
if (!$YourCookieThatPreventsThePopUp) {
//Change that to whatever you call the checking popup, since you don't want it to popup on EVERY forum page
$loggedincheck = ''; //Fill that in with the JavaScript popup code.
}
}
Then reference $loggedincheck wherever you want - header, or cssinclude so it gets stuck in the <head> of the document.