How is this possible in my multiplayer game where the whole world lives in the program? E.g. var map = [...] and then map[x][y] when I want to use it, var players = [], players.push(newPlayer) etc. How can I share this state in a cluster?
That being said, if you are planning on scaling, you'll likely be moving your map object into it's own process, or a more traditional database, etc. Then whatever node application you have that facilitates the communication between that db/process and your users (via websockets or http) will be the place where clustering will come in handy.
That's the ideal case anyway, its hard to avoid session state with websockets etc, but you can still push a lot down to the database layer
If you want a fun exercise, try monkeypatching your global objects (map, players, etc.) to have getters/setters/etc. that transparently handle the syncing stuff.