code: public MucConfigFormManager setMembersOnly(boolean isMembersOnly) throws MucConfigurationNotSupportedException {
          if (!supportsMembersOnly()) {
              throw new MucConfigurationNotSupportedException(MUC_ROOMCONFIG_MEMBERSONLY);
          }
          answerForm.setAnswer(MUC_ROOMCONFIG_MEMBERSONLY, isMembersOnly);
          return this;
      }
docs: Set if the room is members only. Rooms are not members only per default.
  
  @param isMembersOnly if the room should be members only.
  @return a reference to this object.
  @throws MucConfigurationNotSupportedException