<SCRIPT TYPE="text/javascript">
<!--
function popupform(myform, windowname)
{
if (! window.focus)return true;
window.open('', windowname, 'height=200,width=400,scrollbars=yes');
myform.target=windowname;
return true;
}
//-->
</SCRIPT>

<form method="post" action="a.php" onSubmit="popupform(this, 'join')">
...
...
</form>