From https://stackoverflow.com/questions/33078182/socket-io-number-of-users-in-a-room Perhaps a way to get presence indicators.

new_gist_file_0

//Get the room:
var room = io.sockets.adapter.rooms[roomId] || {};
//Declaration of number of users variable
var numUsers;

//If there are any user connected in the room, create the object
if(room.user===undefined) 
  room.user = {};

//If you join the room and have another opened session, increase the number of sessions
if(room.user[socket.request.user.id] && isConnect)
   room.user[socket.request.user.id] += 1;

//If you leave the room and have another opened session, decrease the number of sessions
else if(room.user[socket.request.user.id] && !isConnect){
    room.user[socket.request.user.id] -= 1;

    //If the number of sessions is 0, delete the user from the object
    if(room.user[socket.request.user.id]===0)
       delete room.user[socket.request.user.id];
 }
//If the user don't have any session and join the room, put 1 session in the object
else if(isConnect){
  room.user[socket.request.user.id] = 1;
}

//Get the number of users from room.user object
numUsers = Object.keys(room.user).length;