A cookout lives and dies on the meat order, and the meat order is the one most hosts get wrong, because you buy raw and serve cooked. The baseline is about half a pound of cooked meat per adult, but bone-in and fatty cuts shrink more on the grill, so the raw amount you carry home is always higher. Start with the BBQ calculator for burgers, dogs, and the full spread, and use the meat-per-person calculator when a single cut like brisket, ribs, or pulled pork is the centerpiece and the raw weight is what you need to nail.
Past the meat, the cookout staples are buns at roughly two per guest, two sides, condiments that always run short, and drinks weighted toward beer. As the group grows, grill capacity rather than budget becomes the limit: cook in waves, hold finished meat warm, and prep sides ahead so the grill is only doing what the grill must do.