code: private CmsGroup checkGroup(String groupName) {
  
          try {
              return getCms().readGroup(groupName);
          } catch (CmsException e) {
              throw new CmsIllegalArgumentException(e.getMessageContainer());
          }
      }
docs: Checks if the given group name is a valid opencms user group.<p>
  
  @param groupName the group name to check
  
  @return the read group