Nodejs保存房间的连接套接字

As far as we know creation of sockets is unique id.when my users create room and invite users to join into that. I cant save and store joined users on rooms.when user joined on room (s)he likes to server store this join and after disconnet and connected again joined automatically to latest joined rooms.how can i store sockets and join automatically users?As far as we know creation of sockets is unique