Cooking rib roast for a large group with minimal time
This is a perfect application for sous vide cooking.  I would pre-sear, bag, sous vide @ 58C (136.5 F) for 5 to 10 hours, then chill. All that can be done ahead of time.  Bring to the site, re-therm for a couple of hours, crank the oven, and finish in a hot oven to develop outer crust.