Expanding My Single Player Game


So i’m the coder of a (well known) PC game.
Recently, we launched a completely new version, with new AI etc.
This is the game, www.slendermansshadow.com
Now, i was looking to expand it to multiplayer.
And i tought, we already have all the code, it can’t be hard.
So is there anyone that would like to help me, or put me on the right path?

This is what i was thinking about:

When you click MultiPlayer, you can click Join Server,or Create Server.
Compare it to this,

But the problem is, everywhere i look, its FPS, but our game isnt
And i cant find a person that tells you Without first having to create the FPS game.

So i hope i asked it correctly, greetings



Ok, what i can do so far, is create a server on 1 game, and join it on the other.
On the 1 (server) It’ll say how many players connected.
But i still have no idea in making me spawn, or anything else…

Okay, the way i did it was by download the M2T Networking Pack (50$).
It learned me everything, Very helpful:p