Friend play suggestion

A number of players frequently grumble about the general lack of players. Really it is a bit of a narrow game niche, we're still in beta, and the game doesn't have a lot of advertising, so that should be expected.

But later on, let's say 3-4 years from now, or whenever sales start falling off a cliff, there will be very few players. Multiplayer will be dead. I have an idea to fix that a bit....

I don't know if it has been done before....:

Invite a Steam friend to play the game. Then they can download the game for free, but make their copy only work if you invited them to your lobby.