Chances are you've been "cc'd" on an e-mail message at work. But is it okay to reply to the group if your name is not in the "to" field?
Typically, the "CC" (carbon copy) is to keep you in the loop of a conversation even if you're not directly addressed in the message. Whether or not you reply should be determined case by case. If your participation is relevant to the conversation, then a "reply all" might be in order. But if you have nothing to add or the sender included you as a second set of eyes, consider sitting out and just being a good listener.
Listen to this episode of Awesome Etiquette for a deeper look at "CC etiquette."