code: public void addWorldToGroup(String groupName, String world) {
          if (!groupName.equalsIgnoreCase(DEFAULT_GROUP_NAME)) {
              if (list.containsKey(groupName)) {
                  list.get(groupName).addWorld(world);
              } else {
                  WorldGroup group = new WorldGroup(groupName);
                  group.addWorld(world);
              }
          }
      }
docs: Add a world to a group
  
  @param groupName the group name
  @param world     the world to add