title: How to let 2 socket join the same room to chat (socket.io)
body: <p>I have a problem but researched many days but still do not resolve it.
  I do project chat user to user socket.io and nodejs.
  I am user A, when I click a user B on list user on app, It will let user A and B join the same room (<code>set roomname is (IdOfUserA)_(IdOfUserB)</code> ).
  How can i do the same on server-side, please help me or show me the post with similar problem.
  Thanks very much.</p>