How to join socket.io to common room in server side?

Question

I have next code on client:

Socket.notif.to(eventNameListen[i]).emit('public', { channel: eventNameListen[i] });

What code must be on server side, that will add current user to room eventNameListen[i]?

I tried:

socket.on('public', function (data) {
     // ???
});