Get 3 people who know how to play. Start up a match in Dead Center (the car level). When you are the survivors, have two people go to the top floor and work their way down to the bottom, throwing the gas cans down to the other two members of the team. For the two filling the car, throw pipe bombs when you hear the horde coming. Keep the gas cans out of the spitter acid as they will blow up and your teammates will have to go back for them when they respawn.
When you are the infected, smokers and jockeys immediately go to the third floor and wait for survivors to come to you. When they do, gang up on them. Boomers wait behind the car until a survivor show sup with a gas can and puke on them. The resulting horde should keep them from emptying the can. Chargers wait by the elevator until the survivors go for it, and then charge all the way down at them by the car. You should be able to grab them and carry them to the far wall which is hard to hit by other survivors upstairs. Spitters should sit on the second floor above the car and spit 2-3 seconds before a survivor gets close enough to empty their cans. Hunters should try to catch a lone survivor or gang up with a smoker or jockey on the gatherers.
As the survivors you should have no problem getting 16 cans each time, and then you just need to keep the opposing team from doing the same. Keep in mind hat playing against teams of people usually means they will be well coordinated together, and is much more difficult than random matchmaking.