I guess you could use some kind of ELO rating system. Everybody starts off with a certain rating, e.g. 1500, and then when people play, they gain points (when winning) or lose points (when losing) depending on the ratings of their opponents.
You could use the formulas described at the wiki page, but any implementation of the general idea should do: before playing a game, determine a probability for each player to win the game (depending on their current ratings, and number of players). The less likely you are to win the game, the more points you gain if you do. And vice versa, the less likely you are to lose the game, the more your rating drops if you do.