A large pile of people of mixed sexes, cuddling and or heavy petting each other, usually with clothes on, but sometimes without. Not an orgy, usually done only by people who love and/or trust each other. Not always sexual, sometimes just a physical expression of love or endearment amongst multiple participants.
Originates from the carefree innocent love and romping play that puppies indulge in before they "know better."
So Sam and Lisa were in the corner cuddling when Sarah, Mike, John, and Tanya all joined in and it turned into a real puppy pile.
A puppy pile is when a group of inebriated adults decide it's a better idea to jump in the closest bed available, with whatever lack of clothing they still have on, and cuddle like a pile of puppies, until sobriety catches up with them, and they individually sneek out the back door.
*drunk in hot tub*
'Leona' - "we're out of beer! Let's go make a puppy pile!"
'JoJo' - "That sound's amazing! What's a puppy pile?"
'Leona' - "You, Z, your husband and I go crash in you'r bed!"