code: public void addUser(UserDefinition userDef) {
          String userID = userDef.getID();
          Utils.require(!Utils.isEmpty(userID), "User ID must be set");
          Utils.require(!m_users.containsKey(userID), "Duplicate user ID: " + userID);
          m_users.put(userID, userDef);
      }
docs: Add the given {@link UserDefinition} to this tenant definition's list of defined
  users. If a user with the given name already exists, an InvalidArgumentException is
  thrown.
  
  @param userDef   New {@link UserDefinition} to add to this tenant definition.