def get_user(id)
      return @users[id] if @users.key? id
      user = User.new id
      @users[id] = user
      broadcast(:added, user)
      user
    end
def get_user(id)
      return @users[id] if @users.key? id
      user = User.new id
      @users[id] = user
      broadcast(:added, user)
      user
    end
Get the user instance for a specified user ID.
 If an instance does not exist for the user, one is created and returned.

 @param id [String] The user ID to get an instance for.
 @return [User] An instance of User for the specified ID.