As an additional note, in case the above writeups do not make it clear, anyone who happens along is permitted to try to "slap in" and join a game in progress, as is a player who runs out of cards before the game is over.
The problem that becomes obvious after playing for an hour or two, however, is that those 'phantom' players, having nothing else to do, are likely to either slap in regardless of the card played, or hover their hands right over the pile. In the same way, it is somewhat common for the player who plays the card making a pair to spot the pair (or remember the order of cards in andvance) and immediately slap before anyone else can get their hand over.
My friends and I usually play with rules to deal with these two situations. The player who plays the card making a pair, under our rules, must move his or her hand away from the pile before attempting to slap in a separate motion. Additionally, any player who slaps when no pair is showing has to put his/her top 1 or 2 cards, face up, under the pile, to be taken by the next winner; cardless players owe those cards to the pile when they next slap in, and place them under the first card they lead out.
Any player, under our rules, is free to hover his/her hand over the pile, but only at the risk of having an opponent slap it down onto the pile, thereby earning the player the above penalty.
I understand that some players feel that cardless players who slap in incorrectly more than a certain number (usually 2) of times should be out for the rest of the game. Personally, I neither play nor teach the game this way, since it's frustrating for everyone to have players just sitting around for the next game to start, and it's very hard to judge when a false slap is an honest mistake.