1 - Create a new GAL named Contso GAL.
2 - Create an address book policy (ABP) named Contoso-AD that uses the GAL.
3 - Assign Contoso-AB to all the mailboxes of the Contoso users.
Reference:
https://docs.microsoft.com/en-us/exchange/address-books/address-lists/create-global-address-list
https://docs.microsoft.com/en-us/exchange/address-books/address-lists/address-lists