In order for teachers to share gradebooks, you must give each teacher the role of "Principal". The Principal role simply allows one faculty to view the classes of another.
To make a faculty a principal:
Once the faculty is added to the Portal (see instructions for adding faculty accounts) click the gear icon next to his/her name. Select Change User Type > Make Principal. Then, go to Academic > Grade Book > School Options > Principal Teachers and the "Principal Grades" tab. At the top left, choose the principal's name from the drop down. At the top right, choose all teachers that you would like them to have access to, including the principal's own name. Repeat for all principals.