To grant access to your class go to the Classes screen. Next to your class there is an Access button - click that. The dialog box that comes up will ask you to search for the teacher or teacher assistant that you want to give access to. You can enter either their first name or last name.
Once you locate the teacher you will see a drop-down box next to their name that allows you to grant them View Only or Full Access. With View Only they will be able to see the class but not change anything. With Full Access they can manage the class just like a teacher.
Note that when there are two or more teachers with Full Edit access to a class, a primary teacher should be chosen. That teacher's name will appear on the report cards. To select the primary teacher, go to Settings for the class and click the radio button next to the primary teacher's name.