public function addGroupSigner($name, $email, $signer_index, $group_index_or_role = 0)
    {
      $signer = new Signer(array(
          'name' => $name,
          'email_address' => $email
        )
      );

      $this->signers[$group_index_or_role]->$signer_index = $signer;

      return $this;
    }