Custom Nodejs Server Unity Client

Hi community,

i'm implementing a custom chat server with nodejs and socket io.
basiclly ,players gonna chat only in private , so the user need to select another player to chat with him.

actually this is what i implement, but i didnt find a way to tell the second user to join the private room.
this is my server code

Code (JavaScript):

1. socket.on('connection', function (socket)
2. {
3.     console.log("client Unity connected ");
4.  
5.  
6.     socket.on('registerChat', function(data)
7.     {
8.         roomId =  data.rev+data.me;
9.  
10.        
11.             socket.join(roomId);
12.                 console.log("client Unity Joint room ", data.rev+data.me);
13.                     socket.broadcast.to(roomId).emit('successReg',roomId);
14.     });
15. });

this will add only the player who emit the registerChat event.
my question is how to add the receiver to this room dynamically