code: public T groups_getMembers(Number groupId)
      throws FacebookException, IOException {
      assert (null != groupId);
      return this.callMethod(FacebookMethod.GROUPS_GET_MEMBERS,
                             new Pair<String, CharSequence>("gid", groupId.toString()));
    }
docs: Retrieves the membership list of a group
  @param groupId the group id
  @return a T containing four membership lists of
  'members', 'admins', 'officers', and 'not_replied'