shrimp, Brush shrimps with oil, season with salt and pepper and grill 2 minutes on each side, until shells are hot pink and shrimp is opaque. .Larger shrimp are best on the grill, directly on the grate. Smaller shrimp can be skewered or cooked on a grill pan. Peeled and deveined shrimp are the easiest to work with but if you're feeding a crowd, shell on shrimp will slow them down. At least 1 pound per 4, shelled.
crab legs .
clams, 5-8 minutes, discard any that do not open
scallops, grill, uncovered, 3 minutes on each side or until scallops are opaque, basting frequently with marinade. Easiest to handle on skewers

I would allow at least a quarter pound shrimp, 1/2 pound crab and 5-6 combined scallops and clams for serous fish eaters. Add about 1 pound sausage for each 3-4 sausage , his will ncreate the extra needed.

You might enjoy these off-site crab guides:
delishably.com/meat-dishes/A-guide-to-Crabs-Crabmeat-and-Crab-Legs

www.quora.com/What-are-different-types-of-crab-to-eat-How-do-they-compare

Crab grilling here:
hubpages.com/food/Culinary-Arts-Grilled-Crabs-or-Barbecued-Crabs

Parboil the onions too, and don't forget to grill lemons- up o one per person.