You need to purchase the game once per player. It's in your EULA. Does no one even read those? -.-
The problem is, I bought the game before it was released. Things can change without notice in those instances. I was under the assumption you could do LAN play which, in the case of the other two Stardock multiplayer games, means one copy for multiple people. I think it needs to be advertised when a game will have significant deviation from the norm. I paid a lot of money and waiting a long time for the release of a game I will now never ever play as the only reason I purchased it was to play it with my sister, and I can assure you, she won't be buying it.
I stopped buying games outright about a year ago because there's never any indication anymore if a game requires you to purchase multiple copies just to play with siblings or friends in your own home. I even had a game that only allowed for one save slot so my sister was unable to play without messing with my progress. Both my friends and family will definitely not purchase games to play a few hours with me. In my opinion, this model harms the gaming industry as a whole because it requires players to be gamers. It omits the possibility of casual players getting a chance to play the game--people who probably wouldn't purchase it--causing someone like me, a veteran gamer who prefers multiplayer gameplay, to not purchase the game in the first place. If someone like me stops purchasing games because of this, that should be a pretty good indication it's the wrong way to go allotted others in my situation are doing the same.
Hopefully this changes, or I'm gonna see about refunding my purchase.