Pile On - "sexual definition" - to have more than 2 people join together on a bed, rug, or elsewhere for the purpose of engaging in group sex or group sexual intercourse.
Usually associated with group sex, swinging, swingers, the lifestyle and other free-thinkers.
Linda and Jim were fucking naked on the bed. Debbie got undressed and joined them. Then John walked in, got naked and asked if he could pile on. They all shagged and reshagged for hours. The swingers piled on each other all night.
Usually found in a hardcore show, a pile-on is when everyone rushes from the pit to the front of the stage and jumps on the shoulders of the person in front of them. This is done mostly during a sing-along where the vocalist gives up the mic to the crowd, much like a gang vocal.
Dude that pile-on during Mutiny was fucking croosh!