code: public static Map<CmsUUID, CmsUser> addExportUsersFromGroups(
          CmsObject cms,
          List<String> groups,
          Map<CmsUUID, CmsUser> exportUsers)
      throws CmsException {
  
          if ((groups != null) && (groups.size() > 0)) {
              Iterator<String> itGroups = groups.iterator();
              while (itGroups.hasNext()) {
                  List<CmsUser> groupUsers = cms.getUsersOfGroup(itGroups.next());
                  Iterator<CmsUser> itGroupUsers = groupUsers.iterator();
                  while (itGroupUsers.hasNext()) {
                      CmsUser groupUser = itGroupUsers.next();
                      if (!exportUsers.containsKey(groupUser.getId())) {
                          exportUsers.put(groupUser.getId(), groupUser);
                      }
                  }
              }
          }
          return exportUsers;
      }
docs: Returns a map with the users to export added.<p>
  @param cms CmsObject
  @param groups the selected groups
  @param exportUsers the map to add the users
  
  @return a map with the users to export added
  
  @throws CmsException if getting groups or users of group failed