Good idea, you will be a hit.   You will need around 20 lbs. of meat, and that is hard to cook without a very big roaster.

Try to get 2 roasts to make it work right, if you plan cooking time, thermometer and resting time you can make it work.

I hope you are close to town, and don't worry, the others will be very happy with the results, no matter what.  

Good luck and tell us how it worked out.