def add_client(self, client_identifier):
        """Add a client."""
        if client_identifier in self.clients:
            _LOGGER.error('%s already in group %s', client_identifier, self.identifier)
            return
        new_clients = self.clients
        new_clients.append(client_identifier)
        yield from self._server.group_clients(self.identifier, new_clients)
        _LOGGER.info('added %s to %s', client_identifier, self.identifier)
        self._server.client(client_identifier).callback()
        self.callback()