Launch GAT+ with your Super Admin or delegated auditor account.
Navigate to Classroom audit.
Apply filter and find only the classrooms you want to add additional teachers, then export the list.
To add teachers to all classrooms simply export all your classrooms.
In Classrooms tab click on the Export button and export to Google Sheet
In the exported Spreadsheet, add the new Teacher email separated by (comma)
Add the teacher to all the classrooms, you can use a spreadsheet formula to add them to all rows of Teachers.
Apply the changes and close the sheet.
Click on the Import button and import the changes back into GAT+ and your Classrooms.
A new window will be displayed where you can see the changes done.
In this case, you will see the changes for Teachers row, what teachers are (before) and the new added teachers (after).
When happy with the changes, click on the Accept button to proceed.
When the Import completes successfully, the Teachers will be added to all selected classrooms.