Im having a party and plan on having about 15 people, how do I calculate how much meat (lbs) should I get... or at least is there a way to calculate per person what I need to order?

Thanks

Well if I'm invited you'll need about 20Lbs.

Honestly there isn't really a hard and fast rule, but it really depends on how you intend to serve it.
Are you thinking a full sit down affair or buffet.
How generous do you want to be, etc...

When I made a bone-in prime rib roast, I always take the number of people and put one and a half people per bone. That always yields plenty - because there's people who eat tons, and people who eat a little bit. (I also make delicious sides too... so I could even get away with making a bone or two less, but I like some leftovers).

Half a pound per person. Rule of thumb for catering expensive meats and then add an 8% waste

Two people per bone

Depends if you use bone-in or just the rib with no bone

Standing rib roast with no bone, figure 10 oz per person
At 10oz pp

You would be safe with a 10lb rib no bone, but of course can always go up from there

Usually you will see 8, 10 and 12 oz portions in restaurants