Open a class and select the "Teachers" sub-tab from the left-hand menu.
Next, click on the "Add Teacher" button in the upper right.
A popup window will appear with the list of teachers in your organization. Place a check mark next to the teachers that you would like to have access to your class.
If you need to add a teacher that is not in your organization yet, please first add them to your organization by following these instructions here.
Both Admin and Teacher roles have the ability to add teachers to a class.