public function ReduceInviteCount($UserID, $ReduceBy = 1) {
      $CurrentCount = $this->GetInvitationCount($UserID);

      // Do not reduce if the user has unlimited invitations
      if ($CurrentCount == -1)
         return TRUE;

      // Do not reduce the count below zero.
      if ($ReduceBy > $CurrentCount)
         $ReduceBy = $CurrentCount;

      $this->SQL->Update($this->Name)
         ->Set('CountInvitations', 'CountInvitations - '.$ReduceBy, FALSE)
         ->Where('UserID', $UserID)
         ->Put();
   }