How to Add Co-Teachers:
Open the classroom you wish to add a teacher to and go to the "About" page, then click on the invite teacher box and enter their email address.
Now when you start typing in a new assignment it is automatically saved and a "Draft" section appears at the top of your classroom stream. Only teachers can see the "Draft" section until you post your assignment to the class.
These updates should really increase the usefulness of Google Classroom. For more detailed information about the new features, click here to view the Google Blog post.