Bungie.net has a nice piece up about Halo 3's campaign scoring feature.

Campaign Scoring can be enjoyed by one to four players. It works the same whether you’re playing alone or with friends but it really gets fun when a group of players compete against one another to Finish the Fight and try for the highest score. However, there are trade-offs to be considered – the amount of enemies in the game does not automatically scale with the introduction of more players. So, four people will basically have ¼ as many kills as they would if they played the same settings by themselves. Of course you are also a lot more likely to die, which also will hurt your point total. On the other hand, being a particularly devious player might mean that you can leech a lot of free points and steal kills from your allies while they do all the work. Campaign scoring is fun in all instances but for me there’s not much better than fighting side by side with three buddies and having it all come down to whoever gets the final headshot on the Chieftain to decide the victor.

Be sure to check it out!