In order to cook a roast to "doneness" it has to be cooked without stopping. If you stop part way through the cooking process, the internal temperature will drop to normal or just warm. Later on, attempting to finish cooking it at some place else, in order for the internal temperature to get back up to finish cooking, the outside will over-cook. You'll only end up with a raw center and a burned crust.
Throughly cooking it and then reheating (microwave on low - thus avoiding the busy main oven) individual slices when you get there, will ensure a satisfactory doneness and please everyone.
Have a nice time!