Teach your toddler the grab sign when you observe some grabbing behavior, whether by them, other children, or (hopefully not!) some adults. Then follow this up by teaching that grabbing is not proper behavior when it involves fighting, hurting others, or taking things without permission, and so on. Reinforce the good behavior instead, focusing on sharing, taking turns, and joyful giving.
Teach your toddler this sign in conjunction with the observance of grabbing behavior. Grabbing your keys or bag when you are hurrying out the door is fine, but grabbing something from someone else is not good behavior.