Managed to sort out the this month bit...
altered the code to this: (hope this is ok to alter code, if not please remove this post and except my appologies :erm
Code:
$usersid = $this->post['userid'];
$refcount = mysql_query("SELECT username FROM " . TABLE_PREFIX . "user WHERE referrerid='$usersid' AND MONTH(FROM_UNIXTIME(`joindate`))=MONTH(now())
");
$referrals = mysql_num_rows($refcount);
Baisicly added the AND clause on the end and it seems to work great.
I was also wondering if I only want certain user groups for this to work on (usergroup of the poster not viewer) and I put an conditional statement in my posbit will the queries still run but wont show or do they not run at all.
Thanks again for a great hack