To grant access to a business manager, follow these steps:
Open your Business manager
Using the left-hand menu, navigate to Users, then People.
Click the blue Add button.
Enter the Email address you wish to grant access to.
Choose if you want to grant Employee access or Admin access.
In most cases, Admin access will be more relevant, as Employee access is very limited.
Choose the assets you wish to grant access to.
If you grant Admin access, this step isn't necessary, as they can grant access themselves.
The user you granted access to should receive an invitation to his Email inbox.