Socket.io join another client to a room

My goal is when A user click on B user, they'll join in a room. I'm holding socket.id in array for all clients.

//CLIENT SIDE FOR A//
$('.chat-start').on('click', function(event) {
        event.preventDefault();
        var room_id = '72xaz132s'
        var target_id = $('.target').data('id');
        socket.emit('force join room', room_id, target_id);
    });

//SERVER SIDE FOR A AS AN EXAMPLE//

var clients {'customIDA' : socket.id for a, 'customIDB' : socket.id for b}

socket.on('force join room', function (roomid, customIDB) { 
        socket.join(chat_id); //CLIENT A JOINS ROOM//
    })

Client A joins room without problem, but how can i add also client B on same room ?

In io.on("connection") you can store socket object with user id as a object

{"socket": socket, "id": user_id}

to array. Then find index from array:

let user_index = users.findIndex(user => user.id == user_id);

And finally join to room.

let socketB = users[user_index].socket;
socketB.join('room');