Grill grates aren’t the most supportive surface for burgers, and grilled burgers are gonna shrink up something fierce, so count on 1/3 - 1/2lb burgers being the most stable and bun-appropriate post-cooking (pat fairly flat, but still healthy/thick enough to not crumble on the grates, pressing down a thumb into the middle, to account for shrinkage). If everyone gets a single (they’ll be kinda fat no matter what–again, can’t pat ultra flat like you might for skillet-cooking), then you need at least a pound for every three people. If folks are gonna double up or go back for seconds, increase accordingly.
You can supplement with all beef or chicken/turkey hot dogs. Just make sure to check how level your grill is with something round and roll-y ahead of time. It’s prettier to have the grill marks crossing them perpendicular, sure, but if your grill’s tilted enough that you have to spend 80% of your time fighting the hot dogs to stay in place and not roll off, you’re better off just wedging them parallel to the grates and accepting a little less beauty.
Don’t forget to grill your hamburger buns especially–little softened butter and on they go. Just keep an eye on 'em.
Grilled corn is excellent. Consider grabbing some mayo, garlic, paprika, and cotija cheese so guests can lather up some elote for themselves. Asparagus is also great, but cooks pretty fast; again, worth watching. Long, on-the-bias slices of squash or zucchini is nice, as well. Eggplant, if you salt and drain ahead of time.
If you’re not the only one manning the grill, or even if you are and wanna be safe, consider getting something to visually and immediately ID the meat-utensils vs. vegetarian-utensils. We kept mixing them up and having to go inside and wash them last weekend, d’oh.
Another seafood option is some marinated shrimp (bit of garlic, oil, lime, chilies, salt and pepper) on skewers. Very tasty.
If you want more fun world flavors, consider marinating a couple of pounds of chicken in half a cup of greek yogurt mixed with 2 tsp salt, 2 tsp black pepper, 1 tbsp paprika, 1 tsp chili powder (pure, not the cumin-chili-pepper blend–cayenne is a good substitute–and increase or decrease based on diner preferences), and either 2 tsp cumin and 1 tbsp coriander, or 1.5 tbsp of curry powder, plus the juice of a lime. Marinate for 1-4 hours. Grill. If breast, chop into chunks and skewer. Otherwise, thighs and drumbones work great, too. Delicious chicken tikka.
Baked potatoes are a nice and easy side for you–bake a couple of sheet pans-full ahead of time and toss out with knives, butter, sour cream, cheese, chives, maybe some faux-bacon, etc. in a “bar” setup like your burger toppings.