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

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

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