In most cases the situation you just described will result in a lost fight for your team. In a situation like this you cannot count on your positioning alone to win a teamfight but you can position yourself so you don't die and sometimes you can even get 1 or 2 kills.
Basically the thing you'll have to look after is another teammate that can tank for you. It doesn't matter if he dies but you have to survive. The adc should have the highest sustained damage output in a teamfight and if you follow this rule your chances of survival and also killing a target increase greatly. Note however that you shouldn't stick to the tank just because he's the tank. Sometimes it's better to focus on an enemy that you can kill alone or with another player regardless of who the friendly and enemy player is.
The best thing to do when scattered is retreating as a team. Let them have their baron. Your chances of killing the enemy team are a lot higher in a well positioned 5v5 or 5v4 in your base while they have baron than if you are fighting while you all are split up in the jungle.
Generally as an AD Carry you should stay behind everyone else and you just kite and kill whatever comes in range. Don't try to kill the Enemy ADC or APC. Try to survive tanks and assassins running after you and if you are still alive then you can kill the others with ease. Your job is bringing high sustained damage to the teamfight (this includes killing tanks). Occupying or killing enemy high priority targets is job of the tank and the assassins (or bursters in general).