Some knowledge of how to play Blackjack is helpful. Play is just like regular blackjack, with number cards being worth their numbers (deuces worth two, nines worth nine, ect), face cards worth 10, and aces worth either 1 or 11, the goal being to reach 21. Deal passes to the left. Dealer must hit on 17 and freeze on 18.
The drinking rules are as following.
1) if a player busts, he must drink.
2) if any one player gets a blackjack (a face card and an ace) everyone else must drink.
3) If a person builds up to exactly 21, then they pick who drinks.
4) Winner of hand picks who drinks, if no one builds up to 21.
After deal passes around the table once, it is almost a guarantee that the number of players will have doubled, at least in your perceptions.