[Beorne]

Ok, but I think that a "group" licence with, let's say, 5 or 10 free users with a slighly raised price would be a good idea to greatly increase the market of the game. It is not so easy to convince five or six friends to pay 50 euros. Playing solo is, imho, simply no fun.
Yes, there would be the piracy problem, but I think this can be handle by the paper only manual, it's no sense to play a such complex game without instructions.
Lastly, I think the Space Empires game series sell well enough without serial number.