code: def add_member(self, address, **kwargs):
          
          path = 'membership/%s' % self.id
          kwargs["address"] = address
  
          if "group_name" not in kwargs and self.default_name:
              kwargs["group_name"] = self.default_name
  
          response_data = self.api.request(path, kwargs)
          if 'user_id' in response_data:
              user_id = response_data['user_id']
              return FiestaUser(user_id, address=address, groups=[self])
          return None
docs: Add a member to a group.
  
          All Fiesta membership options can be passed in as keyword
          arguments. Some valid options include:
  
          - `group_name`: Since each member can access a group using
            their own name, you can override the `group_name` in this
            method. By default, the group will have the name specified
            on the class level `default_name` property.
  
          - `display_name` is the full name of the user that they will
            see throughout the UI if this is a new account.
  
          - `welcome_message` should be a dictionary specified according
            to the docs. If you set it to ``False``, no message will be
            sent. See
            http://docs.fiesta.cc/list-management-api.html#message for
            formatting details.
  
          .. seealso:: `Fiesta API documentation <http://docs.fiesta.cc/list-management-api.html#adding-members>`_