Just did one on Christmas Eve. 8# - in the Weber - charcoal, indirect method. Rubbed with a mixture of Kosher Salt, Black Pepper, Garlic Powder (even amounts of each). Grill ran at about 300 degrees - cooked for 3 hours to get to 120 interior temp (but it was still partially frozen when started). It was the best prime rib I’ve had in years 
Of course, slicing with a deli slicer is always nice too!
I had to run through about the same amount of rib roast for our family Christmas party, which was about 35 strong.
I happen to have a deli slicer though, and it certainly made the job MUCH easier than if I had to do it with a knife. 
(My FIL is/was a chef, and he had an extra… since I married his daughter, I got his spare)
The family was baffled & amused that I happened to have one “lying around,” but was very grateful for it!
The roast came out GREAT. The night before, while panicking that all I had was an old non-digital meat thermometer, I happened to remember that my oven had a temperature probe that I’d never used. So, I turned the oven on to 350*, set the probe for 130*, and planned to remove it from the oven in 5-6 hours. Imagine my surprise when it beeped after only 2.5! No way it could be done, I though to myself, so I reset the probe to another place in the roast, reset the oven, and dang it if it didn’t record over 130* already! So, I turned off the oven and let it sit for another hour before my curiosity got the best of me and I cut the roast in half. Perfect! Medium, just a hint of pink. Everyone lived to talk about it, but I didn’t want to see it again for a LONG time. I even got a phone call at work today. The family wanted to come over to get the sliced leftovers for sandwiches. Success!
I hadn’t realized that convection roasting would cut the time in half, but it did a fantastic job, and it worked out great. Thanks, everyone, for your help!