I can think of a few items you could cook on your new rotisserie setup for your Weber kettle that would feed your group of 10-15 people.
My pictures and information will show these cooks in an EZ-Que rotisserie basket but the same thing can be done on your spit rod setup very easily.
This first cook will be a Cumberland Gap boneless ham and these weight about 7-9 pounds and sliced thinly will easily feed your group.
This next cook will be roast beef and I usually do two of them at a time and two will fit on your setup. These roasts will weigh about 3-4 pounds each and sliced thinly will easily feed your group.
The last cook and I don’t have any pictures on the Weber but you could cook two turkey breasts on your rotisserie setup at one time. These will weigh about 7 pounds each and this as well will feed your group.
Hope this helps...