code: def add_users(self, users=None):
          
          if not users:
              raise ArgumentError("You must specify emails for users to add to the user group")
          ulist = {"user": [user for user in users]}
          return self.append(add=ulist)
docs: Add users (specified by email address) to this user group.
          In case of ambiguity (two users with same email address), the non-AdobeID user is preferred.
          :param users: list of emails for users to add to the group.
          :return: the Group, so you can do Group(...).add_users(...).add_to_products(...)