NODE.JS accessing shared variable in socket.IO event handler

I am doing an experimental online poker game using NODE.JS and socket.IO. The game requires 3 players to join to start. I use socket.IO to listen to the connections from joining players. Whenever there are 3 players coming, they will form one group. Currently I use some shared variables to do this. But if there are lots of players coming in at the same time, I am fear it will cause synchronization problem. As you can see from the code snippet, the players, groups, groupNumber, comingPlayer, clients are all shared between multiple 'connection' event handlers. So when one event handler is executed and another event handler got scheduled by the V8 engine, it may corrupt the states of these shared variables.

I have done some research using Google but didn't find satisfactory answers. So I posted here to see if any expert can help me. Thanks in advance!

var clients = {}; // {"player1": 1} {"player2": 1} {"player3": 1}
var groups = {};  // {"1": ["player1", "player2", "player3"]
var groupNumber = 1; // the current group number
var comingPlayers = 0; // a temporary variable for the coming players
var players = []; // a temporary array containing the players which have not formed 1 group

socket.on('connection', function(client) {                                                                                                  
    sockets[client.id] = client;
    players.push(client.id);
    clients[client.id] = groupNumber;
    comingPlayers++;
    if (comingPlayers === 3) { // now there are 3 players which can compose 1 group
        groups[groupNumber] = arrayClone(players);
        gamePlay(groupNumber);
        players = [];
        groupNumber++;
        comingPlayers = 0;                                                                       
    }          
}