Legal Question in Criminal Law in California

Merely being present would not make you an accessory. But you might have done something more that made you one. Even if you didn't, it might appear as if you did.

An accessory is somebody who helps a person who committed a felony to escape, hide evidence, etc.

You can't be an accessory to a misdemeanor, no matter what any other answers say.

If you're asking if you could also be charged (different theory, different label) because you were with your friend when they stole, then that depends. They would have to prove that you did something. Just being there isn't enough. Being a lookout or assisting in any other way may be enough.

If you're charged, get a lawyer.