don't forget about shrinkage from the meat. the amount of the poundage of the meat will not yield what you bought, once it is cooked. for quick cooked meats like steaks, it's not that big of a problem, but for roasts, or for anything that will be slow cooked for several hours, the yield is much less. If the roast is the only main course you will be serving, plan on everyone having at least 2-3 slices at about 6-7 oz each (remember that people eat more for special occasions). If you are supplementing it with other main courses, then plan on everyone having at least 1-2 slices at the same portion size. It also depends on who is carving the meat, and how generous they are cutting it. If you figure a roast will have about a 60-70% cooked yield factor from when you bought it (i.e. 10 lb. uncooked roast = 6-7lb. cooked roast), then for every 6 oz. cooked portion, you're going to have to count it as a 10 oz. uncooked portion when buying your meat. Finally, taking all these factors into consideration, if you are inviting say, 100 people, prep for 120.
"Boldness has genius, power, and magic in it."
Johann Wolfgang Von Goethe