[saf-t scissors]

I'm not familiar with the coin subscriptions, but finding members of a certain usergroup, modifying their accounts, and continuing to the next is fairly generic.

You'd need to create a query that accurately describes the users you want

$myusers = $vbulletin->db->query_read("
SELECT userid, (... etc)
FROM " . TABLE_PREFIX . "user
WHERE usergroupid=2 (or whichever group you want)
");

then just:

while ($myuser = $vbulletin->db->fetch_array($myusers))
{
do your thing
}

I'd recommend downloading a few different scheduled task add-ons and examining their code to get a better understanding of how to accomplish your goal. A bit of time with some example code and a test installation of your board, and you should have a working script in short order.

Hope that helps.