In reply to Heike:
I enjoy putting a load of effort into cooking for a small number, and have done some big (effort) meals for groups up to 20. However, a bbq has a lot of variability and for 50 the above replies are right that you need ruthless simplicity.
Hog roast is an amazing call, buffet is good, but if you want to still go for a bbq (it's just fun isn't it?) then I've done it really easily and simply. Buy burgers, just burgers, and get fairly thin ones. They all cook fast and at the same rate and beef isn't as bad as some other meats if underdone. This means you can start grilling as soon as they're is heat, as you can probably cook them before they burn. Find someone who likes tessellation problems... I'd get 100+ burgers but I hate under catering. Big buns and then spend what you like on interesting things sauce (ketchup, mayo, mustard, mango chutney), cheese (cheddar, blue cheese, anything a bit different), gherkins, slice up tomato - look at a GBK menu for inspiration. Get some bags of pre-prep'ed salad. Bulk it out with some boiled new potateos with butter, or turn it into a big chunky potato salad (onion and celery diced up adds some flavour, can mix mayo and plain yoghurt if you don't like just mayo). Garlic bread done in the oven is a good filler. Crisps and dip, etc etc.
I've never had to worry too hard about veggies; have done big burger sized mushrooms with a bit of garlic butter on them. Everyone loves haloumi too.
You can almost certainly deputise the bbq to someone (someone is always keen) and rotate it round a bit. If not, once you get a heat on the burgers will fly off fairly fast and once you get a rhythm going you don't have to watch over it all the time.
Marshmallows are good done over the coals if you're going late into the evening.
Hungry now...