code: def add_client(self, client_identifier):
          
          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()
docs: Add a client.