Du kan använda mysql_insert_id()
$query1 = mysql_query("INSERT INTO events (name , location , date) VALUES ('".mysql_real_escape_string($name)."' , '".mysql_real_escape_string($location)."' , '".mysql_real_escape_string($date)."')");
$insert_id = mysql_insert_id() ;
$query2 = mysql_query("INSERT INTO attendance (event_ID , member_ID) SELECT {$insert_id}, members.member_ID FROM members") ;