There's some serious commitment involved in picking out prime rib at the grocery store, so you'll want to get your details straight before heading out to shop.
In addition, keeping ribs intact makes portioning easier: A single bone per every two people is a reasonable ratio. In terms of weight, estimate about a pound per person.
As with steaks, prime rib is also delineated by the top beef grades: prime, choice, and select. For the most flavorful roast, track down a prime-graded prime rib.