From the introduction:
This is a game for an even number of players, around six to twelve, but more should work just fine. (If you wind up with an odd number of players, that’s not an insurmountable problem; one player will just sit out in each round.) You’ll need a number of index cards, one for each player. One of these cards will have a black spot or mark of some kind clearly drawn on it. Shuffle up the cards, and hand one to each player; the players should not know which one of them received the card with the spot. The player who received the mark is the first Invader. Their minds have been corrupted by an alien presence, and they will attempt to similarly infect the other, still human, players.