I'm not sure myself whether it involved everyone, (at least in the actual murder), but I think that, at the very least, they all know that someone in their group did it and they're covering for that person. Either by hiding evidence, lying to the police, or trying to place blame on others. At the worse end of the spectrum, they planned it all out together.
It is funny, EastTexasTic, that were were thinking of the same thing! I like your idea that maybe it was a bungled attempt that that's why things got out of hand.