How do I get involved in this community?
Run away while you still can.
Seriously though, you have to buy M&B Warband and then the Napoleonic Wars expansion, then download the mod (free).
Might be worth waiting for the steam sale if it isn't on sale right now (usually goes to 75-80% off).
You should also be aware FSE is going to be releasing Battle Cry of Freedom at some point in the near-ish future. Its a standalone Civil War game from the devs of NW. There's also War of Rights coming out soon, another similar standalone ACW game (by the devs of North and South). Both these games are in alpha and will replace North and South