How can my agency or media group become an admin of my client’s Business page?

Follow these steps to become an admin on a client’s Facebook page:

Step 1:
Log in here: https://business.facebook.com/
Click Settings
Click Users
Click People
Click Add
Invite the Market/Agency email address

Step 2 (once you confirm email):
Log in here: https://business.facebook.com/
Click Settings
Click Accounts
Click Pages
Click on the page and click “Add People”