Depends on the size of your group, but I quite like a whole leg of lamb cooked on the grill. Either:
1: Butterflied leg of lamb
Debone the whole leg (or ask a good butcher to do it) so that you have a huge butterfly shaped lamb steak. Thickness will be from 1/2' on the edges, to about 4" in the middle. Cut a few slashes in the thickest part.
Marinate for as longs as possible (ideally 24hrs) in LOTS of lemon juice, garlic, crushed fresh rosemary, olive oil, salt and pepper. Cook fairly slowly on a grill over a good bed of embers, or a BBQ (Webber style work well with the lid left on).
Finished meat will be well done and crispy in the thinner areas, and nice and pink in the thick central part. So should be something to suit everyone.
Serve with Greek Yoghurt with some fresh chopped mint leaves mixed in it, and sprinkle over lots of bright red Pomegranate seeds.
2: Brazilian Style Leg of Lamb:
This is much more camp friendly but if cooking at night have a good light source to check the cooking.
I was served this at a cattle ranch in Brazil, where one of the cattle hands threw a whole leg of lamb onto a really hot BBQ. I naturally thought that it would be completely burnt before it was cooked through, but he whipped it off 10mins later and just carved away and served the perfectly cooked outer layer (Doner Kebab Style). Then it was back on the BBQ for another short blast whilst we drank more wine, and had some salad etc.
Took a couple of hours to eat the whole thing a layer at a time, but it was delicious.