Probably yes, on the grounds that the cheering tends to encourage the criminal and thus constitutes “abett[ing].” “An aider and abettor is one who acts with both knowledge of the perpetrator’s criminal purpose and the intent of encouraging or facilitating commission of the offense.” People v. Avila, 38 Cal. 4th 491, 564 (2006).
This having been said, convictions based on solely encouragement-by-cheering, without any more tangible help, are apparently rare. Unless I’m mistaken, this theory was tried as to some bystanders in the infamous New Bedford barroom rape case, but they were acquitted. And though I think there’s no legal bar to such a conviction, I suspect that it’s often hard to pin down just who was cheering, and likely sometimes hard to persuade a jury to convict based on cheering alone. [...]