In The Pigman, by Paul Zindel, John and Lorraine are having a party at the Pigman's house, but it gets out of hand. There are about forty people there, some of whom weren't even invited, and of course because there's alcohol and smoking involved, the house is becoming a terrible mess. So the question is, why did the kids even invite so many people?
John is narrating Chapter 13, which is when this party is happening. He tells us:
"We really didn't start out inviting too many kids, but the more Lorraine and I thought of the parties we had been invited to, the more we had to call. After all, it was the first time either one of us had a chance to return the invitations we had gotten."
So basically, they invite so many friends because they'd accepted those friends' invitations to parties in the past, and it's considered good etiquette to host people who have hosted you before (just like it's polite to get someone a gift during the holidays if you know that person is planning to give one to you).