Did Piggy and Ralph participate? The current answers range from "Yes" (but maybe no) to "Probably" (maybe, maybe not) to "Ralph did but not Piggy". I'd like to explain why the correct answer is an unequivocal yes - that they were both involved, both were complicit in the murder, and they both sought to lie about it.
First, let's quickly set the scene. Simon has gone off on his own into the forest, has a delusional episode with a pig's head on a stick (the "Lord of the Flies"), has a fit and passes out. When he comes to, he climbs the mountain and discovers that the "beast" is merely the decomposing body of a parachutist. Meanwhile, Jack and Ralph have fallen out, and Jack has staged a mutiny, forming his own gang of hunters. The hunters kill a pig, raid Ralph's camp and steal fire, and hold a party. Ralph finally gives in, and he and Piggy join the party. Ralph challenges Jack, but a thunderstorm hits, and Jack's response is to summon his hunters to "Do our dance!"
Between the flashes of lightning the air was dark and terrible; and the boys followed him, clamorously. Roger became the pig, grunting and charging at Jack, who side-stepped. The hunters took their spears, the cooks took spits, and the rest clubs of fire-wood. A circling movement developed and a chant. While Roger mimed the terror of the pig, the littluns ran and jumped on the outside of the circle. Piggy and Ralph, under the threat of the sky, found themselves eager to take a place in this demented but partly secure society. They were glad to touch the brown backs of the fence that hemmed in the terror and made it governable.
“Kill the beast! Cut his throat! Spill his blood!”
Roger rejoins the circle, the thunder and lightning are constant, the chanting goes up a notch, and now "out of the terror rose another desire, thick, urgent, blind":
“Kill the beast! Cut his throat! Spill his blood!”
It's in this terrifying and deranged setting that Simon crawls like a beast out of the bushes and into the circle. Beastly Gerbil's answer provides the grisly details of Simon's brutal death.
The next chapter gives us the answer. Piggy and Ralph are alone on the beach the next morning, and Ralph is clearly troubled about what had happened. Finally he says it: "That was murder." Piggy immediately (and "shrilly") tells him to stop talking like that.
"It was dark. There was that––bloody dance.There was the lightning and thunder and rain. We was scared!"
But Ralph says he wasn't scared, he was ... "I don't know what I was". Nicely ambiguous: does he mean he doesn't know what emotion he had felt, or he doesn't know what thing he had become - an animal, a beast, a madman? At this point the reader might also recall an earlier chapter where Ralph and the hunters had first performed their kill-the-pig dance, with Robert as the pretend pig within the circle:
They got his arms and legs. Ralph, carried away by a sudden thick excitement, grabbed Eric's spear and jabbed at Robert with it.
"Kill him! Kill him!"
[...] Ralph too was fighting to get near, to get a handful of that brown, vulnerable flesh. The desire to squeeze and hurt was over-mastering.
Ralph has got form; he knows what he did - what he was - and is in turmoil. "Oh Piggy!" he says, his voice "low and stricken"; "cradling the conch, he rocked himself to and fro."
"Don't you understand, Piggy? The things we did–––"
Another answer cites Ralph's description that Piggy was "outside the circle", but Piggy's "shrill" reaction has all the hallmarks of a guilty boy desperately searching for a defence or an excuse:
"... he had no business crawling like that out of the dark. He was batty. He asked for it." He gesticulated widely again. "It was an accident."
But the clincher is Piggy's final conspiratorial don't-tell-anyone appeal, and Ralph's willing complicity in the lie:
"And look, Ralph," Piggy glanced round quickly, then leaned close––"don't let on we was in that dance. Not to Samneric."
"But we were! All of us!"
Piggy shook his head.
"Not us till last. They never noticed in the dark. Anyway you said I was only on the outside–––"
"So was I," muttered Ralph, "I was on the outside too."
Piggy nodded eagerly.
"That's right. We was on the outside. We never done nothing, we never seen nothing."
The scene ends with the Samneric twins returning with firewood, everyone acting guilty and assuring each other that they hadn't been involved.
The air was heavy with unspoken knowledge. Sam twisted and the obscene word shot out of him. "––dance?"
Memory of the dance that none of them had attended shook all four boys convulsively.
"We left early."