Is it rude to forward an email to someone left out of the email?
This is a bit subtle, since email client software is different for different people. A lot of people replying to an email don't take the time and effort to look at who (all) will get the mail.
In the general case, for a group that communicates frequently, you can, perhaps, establish guidelines. In the specific case, you could ask the professor that sent the email - i.e. A.
But, if the mail is about group work that everyone should see and has no personal elements, then there is little to fear. It was probably just an oversight.