We often turn to Nicole Aloni’s Secrets from a Caterer’s Kitchen for help with these kinds of questions. While written from her many years’ experience as a professional caterer, it is produced primarily for home cooks who want to throw a bash every now and then. Of particular value in this case is the multi-page chart outlining suggested amounts for buffets of varying sizes.
If 50 people have a choice of two entrees, Aloni suggests 28 to 32 pounds of bone-in chicken pieces and 20 pounds of steak will be ample. (Actually, she says individual steaks are best served at smaller events - 12 people and fewer - but since yours is a barbecue, and, we assume, you have plenty of surface area on your grills, we think steak should work fine.)
With regard to the potluck portion, assuming people will have a choice of two or three vegetables, two or three salads, and two or three desserts, plus bread, you want to generate some reasonable combination of the following choices: 6 quarts of fruit salad, 12 to 16 cups of cole slaw, a green salad made with 8 to 12 heads of lettuce, 4 pounds of pasta salad, potato salad made with 10 pounds of potatoes, 8 pounds of this vegetable or that vegetable, and 35 ears of corn.
On the dessert front, she suggests choosing two or three of the following: 2 gallons of ice cream, 4 pies, 6 dozen brownies, 6 to 8 dozen cookies, 4 8-inch layer cakes, and 4 9-inch cheesecakes. Sounds like a feast to us.