[WhaLberg]

Yes, this requires a plugin. Do a search on 'vbmail' function and use it in 'newpost_complete', 'newreply_post_complete', 'newthread_post_complete' hooks. You also will need to get the users who are in the usergroup that will be notified.

PHP Code:

$users = $db->query_read("
SELECT email FROM " . TABLE_PREFIX . "user
WHERE usergroupid = 'x'
");
while ($user = $db->fetch_array($users))
{
// vbmail function..
}