Such silly or goofy behavior is almost always of a good nature, and involves good, sometimes clean fun.
Said behavior will usually happen when all or most people in said group are either bored, hyper, or overly tired.
It can also be identified as the actions done by said group that come about as a result of the friends in such group "getting high off of each other", a phrase used to describe the feeling of being high when around certain friends for a given amount of time, simply because they are awesome and fun to be around and they always have fun together and their company is very much enjoyed.
Shenanigans is also, but less often, referred to as sneaky, deceptive, or mischievious behavior.
*Peter sitting by himself*
*Several friends approach him*
Kim: Hey, man, there's room over there with the rest of us if you wanna join.
Peter: Yes, I'm aware.
Kim: So you wanna sit by yourself like a loner?
Peter: Yeah, I'm fine, thanks.
*Kim and others return*
Leila: What's wrong with Peter?
Kim: Nothing, he just wants to sit by himself, and not join our super-fun shenanigans that we are having.
*All other than Peter (sad face) commence in having a super-fun time of shenanigans*
3) Playing improve games with Nick (happy face). Such games include "freeze", "bang", "atom", "the party game", "psychic battle", etc.
4) Having a dance party outside of the chorus room.
5) Having (and losing) a crab fight with Leila, resulting in accidentally banging your head against a chair.
*from the movie Super Troopers
Mac: Hey Farva, what's the name of that restaurant you like with all the goofy shit on the wall and the mozzarella sticks?
Farva (from other room): You mean Shenanigans?
Mac: *hands O'Hagan the butt of his pistol*
a. A deceitful trick; an underhanded act.
b. Remarks intended to deceive; deceit. Often used in the plural.
a. A playful or mischievous act; a prank.
b. Mischief; prankishness. Often used in the plural.
Tom: I was at a funeral in Virginia all day man, im sorry i couldnt come over.
Jack: I call shenanigans, i saw you walking down the sidewalk yesterday.