normally parents, but it could be die-hard fans etc.
Mom: "Did you hear little Billy's singing?"
Aunt: "Yes his voice really stood out"
*Mother and Aunt only ones clapping still*
Bystander 1: "That afterclap is lame, their kid totally sucks, it was a choir of 200 kids!"
Bystander 2: "Yeah afterclap kills a show sometimes.."
Usually participants are bored kids in school