def update_members(self, group_id, members):
        """
        Updates the membership of the group represented by the passed group id.
        Returns a list of members not found.
        """
        self._valid_group_id(group_id)

        body = {"data": [m.json_data() for m in members]}
        headers = {"If-Match": "*"}
        url = "{}/group/{}/member".format(self.API, group_id)

        data = self._put_resource(url, headers, body)

        errors = data.get("errors", [])
        if len(errors):
            return errors[0].get("notFound", [])
        return []
Updates the membership of the group represented by the passed group id.
        Returns a list of members not found.