Given you say you’re in the final stages yet never having programmed a multiplayer server, I suspect you might be underestimating how much work is involved in adding multiplayer support
Are you using the JS API for cocos2d? If so, you should just try implementing the examples on the socket io website, first. Once you’ve got that working you will be better placed to implement multiplayer in your game.
In very broad strokes: one way of relaying inputs between users is having a central server to which all clients connect. When a user makes a move, they tell the server. The server then sends that move to all other players in the same room who deal with it accordingly.
The multiplayer apps I’ve worked on have either been purely turn-based (easier to synchronise), or P2P real time (no central server).