Setup
Divide players into two teams. The teams need not have the same number of players (ie. 3 versus 2, or 4 versus 3, etc.) Separate the teams about 10 feet from each other, everyone sitting on the floor. Each team will have a cup in front of them which is their goal, plus their own drink. Any standard drinking cup will do.  Fill the cup about half way with water. Distribute bottlecaps among the teams, you will probably need at least 15 caps, but you will be able to play with 5 if need be.  Look in the coushins of your couch, under your couch, under your refrigerator, in your pockets, and near garbage cans wherever you are playing and you will probably find enough.

Play
Teams take turns throwing into the opponents' cup, and players on a team must take turns in order (like in baseball). When it is your turn, select 5 bottlecaps and try to toss them into the opponents' cup. You must throw your cap from no closer than your own team's cup - that is, stay behind your cup. The water in the cup prevents a cap from bouncing out once it scores. If the cap lands in the cup, leave it in there (if you have enough bottlecaps). That way you can tell at a glance what the score is. Otherwise, take the cap out and use it again. If the cap bounces off the floor or a wall and lands in the cup, it still counts. You can not try to block your opponents' throw, even after it bounces. This is called "goaltending" or "basket/beersket interference," and it counts as a goal. If it happens to hit you, let it fall - it is still live. The cap can ricochet off a person and into the cup.

Scoring
The first team to sink 5 bottlecaps wins. If a cap goes into the cup and stays there, it counts as 1. The winner must win by 2, so if the score is 5-4, keep playing to 6-4 or whatever happens to be the final score (n to n-2).

Drinking
When the other team scores a point, drink half a can or bottle of beer. If the cap only hits cup but does not go in, take one drink. (Usually defined by a swig or a gulp) If the cap hits the cup, then the floor, then the cup again, it in effect glanced off the cup twice, BUT that does not count as two hits - take only one drink. Wait until your opponent finishes his or her 5 throws, then do all of your drinking.  Everyone on the team must take the drinks. For example, out of the 5 shots, if 1 went in, 2 hit the cup, and 2 were air-caps (like air-balls), everyone on your team would drink half a beer plus two sips/gulps. (An air-cap has no drinking associated with it) Finish drinking before you make your next throw.

Cup Selection
Try not to use a glass cup, because some glass will tend to crack, chip, or shatter after being struck several times by bottlecaps. The wider the mouth of the cup, the easier it will be to score. Also, if the cup is short, there is a better chance than a cap will bounce into it. The amount of water in the cup also has an effect on the game. Filled to the brim, the cup will add insult to injury by splashing your opponent every time they are scored upon.

Cap Selection
Different types of bottlecaps perform differently in play. When you choose your five to throw, consider choosing 5 of the same type (like 5 Bud Dry (tm) caps) so that you have fewer adjustments to make with each throw. Foreign beers generally were opened with a bottle opener, so the cap will be dented or bent, and therefore won't fly as true. Some caps are too light, and tend to veer off of their original path just by meeting air resistance.

Technique
The most frequently-used method of holding the cap is described here: Grasp the cap between your index finger and your thumb. Hold your arm out towards your opponents' cup. Then throw the cap like a dart. The cup will undoubtedly be lower than your point of release, but still put some arch into your throw. This will increase the chance of a bounce-in or a ker-plunk (equivalent to a swish). Other techniques are the underhand lob and the "coin-toss" flick, which are more difficult to refine.

Special Rules
Sometimes strange things happen and the rule for that may not be known. If such a thing occurrs, just make up a fair, lenient rule that applies for the game you are playing, but establish an offical rule that is in effect for subsequent games. Here are some rules that have been created in such a manner:

If you throw a cap into your own cup, it counts towards the other teams total points, it counts as one of your 5 throws, and the opponent still gets 5 throws their next turn. The first time it happens, consider calling a do-over. It is also fair to say that if that scores the oppenents' fifth (and winning) goal, disallow it, or else play to 6 (or one more than usual).

If you throw a cap into someone's drink and not the goal, it counts as a miss. It also really upsets the person whose drink it is, so while there is no penalty in the game for doing so, there may be other reprecussions! (A precedent was set with Leigh Derrick vs. Mark Buckwalter)

Advanced Play

Stacking
It is a valid move to stack multiple bottlecaps onto each other and throw them at once. If you do, for example, toss two bottlecaps at one time, they both count against your total of 5 for that turn. But if they both go in, they both count towards your teams total points.

Adjusting
Before your opponent throws the first of his or her five caps, you are allowed to move your cup a resonable distance away from where it was last time. This is effective if the other team seems to be zeroing in on the right trajectory to toss their caps. But once they throw their first cap, the only adjusting you can do is to rotate the cup in place, which basically doesn't have any effect if the cup is round.

Force Field
In an attempt to psyche out your opponent, you can pretend to create an invisible force field over the cup by waving your hands over the cup or pretending to zap the cup, like an evil witch might do if she was casting a spell. When you do it, say "Zzzt. Force field." (Courtesy Jason Lewis)

Variations
Of course changing the amount of drinking changes the game. A full beer per score seems steep to me, but go for it. Also play to a different winning score, like 10 would double the drinking for the losing team, but would also take forever.

Tournament Play
Almost any style of tournament lends itself to team Beersketball. When the # of people playing can be divided up into a number of teams which a power of 2, a single elimination format can be applied. With many teams, a round-robin format could be implemented. Also try a double-elimination tournament. King-of-the-hill works as well, where the winning team stays to be challenged by other teams in turn. With King-of-the-hill, the champion teams will inevitably be drinking more because they play multiple games in succession, so there is always a good chance for a new king of the hill team. When there are newbies playing with veterans, consider the $20,000 Pyramid format, in which an individual's score is kept and they play with different teammates each game.