Some friends and I are in the process of building a FPS wave survival shooter. A lot like Killing Floor actually. So the game is going to be mainly a multiplayer game. We currently have a basic framework for socket communication setup and a working world with physics where you can run around in.
So my question is when one is building such a multiplayer game should one build the network system along with the game development or should we create a working game with an entity/level/AI etc systems before adding a multiplayer system?
Your networking code is almost certainly going to be integral to your game engine. Write both at the same time; choices you make for one will inform related choices in the other.