Building a Multiplayer Hide and Seek game, can't figure out the multiplayer portion.

I have built a SP version of a simple Hide and seek game where you find a box for 5 levels. the game was coded in simple pieces of c# (most of which was reused, a lot.) however, I wanted to expand on my game and make it a MP game. the game would probably have to be started from scratch with: New levels, a Networking system bigger levels and a lot more.

any ideas where to start?