I use one medium potato per person (or two small ones, like chats) and always make a couple of extras for leftovers. They make a great lunch the next day. I like to leave the skin on, but you can peel them if you choose to.
To prepare, you'll need to cut the potatoes into bite size pieces. Tear off pieces of foil, about 15cm long to make the parcels. Spray the foil with cooking spray. Place one cut up potato on each piece of foil. Sprinkle with your favourite seasonings. Garlic powder, ground black pepper, paprika, chives and parsley are all good. Dice an onion and sprinkle some over each potato and dot with butter or margarine. Fold the foil to make a parcel around the potato so it won't open on the barbecue. The potatoes take about 15 minutes longer than the meat to cook so put them on first, then cook the steak and sausages.
You can use this same type of recipe for just about any of your favourite veggies: zucchini, red, green or yellow capsicum, eggplant, sweet potato and so on. Use your imagination. Mix up as many different types of veggies as you want for a mixed vegetable parcel.