If a Class Team has been created for you via a sync with Arbor, before they can be used fully, they must be activated.  To do this browse to the Class Team and then you'll notice a banner at the top of all your new class team that shows you how many students are waiting to join the class.

Navigate to the team and select Activate.

A team with a banner and Activate button.

Select Activate again to confirm that you'd like your students to access the team.