Who was that guy at the end ?!

He was on the stairs at the beginning with Dane when the girls walked down the hall. Dane says "That's the plan". Then he was part of the 3 dudes at the 3rd table in the cafeteria scene. (the triplets). He was then part of the 3 outcasts in the woods, wearing the "same costumes as us, no-one escapes". The bulk of the subplot was cut from the film so it's tough to figure out. At least that's what the trivia and all the message threads say.

He was involved, but got away with it.

I am not sure if they were actually triplets or if the two were twins and the third guy was a friend and they people just refereed to them as the triplets. In the movie they reference them as the triplets and then later in the movie as the twins and the "other/third one". From the beginning it was pretty clear that they were helping the others get their revenge but had no intention of committing suicide or turning themselves in when it was over. I think this was because either were not picked on at all and just sympathized with the ones who were bullied or if they were picked on and just coped with it better because they had their brothers/friend to lean on and they had no interest in being friends with anyone outside of the three of them, couldn't care less what other people thought about them and managed to isolate themselves from everyone a long time ago. If they dressed like the rest then witnesses and/or police would just think it was one of the members inside getting revenge running around outside. They also wore the same costumes because they were running around in BFE after dark and the costumes helped decern a escapee from those who were in on it. They didn't want to accidentally take a buddy out. Of the three only one survived and he returned to school. When the kid that plays the hero hears him revving the engine of the bike he realizes that the guy was there helping the others and had gotten away with it.