Update a membership.

        Accept a membership request, or add/remove moderator rights.

def update_membership_users(self, user_id, group_id, moderator=None, workflow_state=None):
        """
        Update a membership.

        Accept a membership request, or add/remove moderator rights.
        """
        path = {}
        data = {}
        params = {}

        # REQUIRED - PATH - group_id
        """ID"""
        path["group_id"] = group_id

        # REQUIRED - PATH - user_id
        """ID"""
        path["user_id"] = user_id

        # OPTIONAL - workflow_state
        """Currently, the only allowed value is "accepted""""
        if workflow_state is not None:
            self._validate_enum(workflow_state, ["accepted"])
            data["workflow_state"] = workflow_state

        # OPTIONAL - moderator
        """no description"""
        if moderator is not None:
            data["moderator"] = moderator

        self.logger.debug("PUT /api/v1/groups/{group_id}/users/{user_id} with query params: {params} and form data: {data}".format(params=params, data=data, **path))
        return self.generic_request("PUT", "/api/v1/groups/{group_id}/users/{user_id}".format(**path), data=data, params=params, single_item=True)