User cannot open shared mailbox in Outlook 2016 - GAL problems

I've got the following problem which really does my head in. 
When a user A shares his inbox in o365 and gives user B full rights to that mailbox, and doesn't show automatically in User B's OWA or Outlook 2016.
OWA: To add the inbox A to user B, we rightclick the username of B in the OWA and select  "Open shared folder". When we start typing the e-mail address of User A, it isn't found. When we send  User A an email and then redo the steps, we can open the Shared inbox and it is shown in OWA.
Outlook: When we try to add Inbox A to User B's Outlook, the error message "The name could not be matched to a name in the address list." and we can never add the shared mailbox to User B.

The address list error message pushes me in the direction of the GAL.
As we're a school we have GAL segmentation enabled and created the following for: 

Staff Addresbookpolicy (ABP): StaffABP which contains: GAL "Org Name", Address lists: "Students", "Staff","Distribution Lists", OAL:"Offline AddressBook", Roomlist "resources"

I guess the error comes forth that the user cannot browse the custom GAL and tries the default GAL, which has no entries. 
My question therefore is: 
How do i populate the custom GAL with all Staff members so that i can find the user to add the shared mailbox in Outlook 2016.

Hopefully my question isn't too vague!