 Login to your Facebook account and navigate to your Facebook Business Page.

From your Page, click "Manage" in the top right.

From the left-hand menu, select "Page Access."

Click "Add New" beside "People with Facebook access."

In the "Assign a New Page Role" section, begin typing the name or email address of the person you want to add as an admin.

Choose the role you want to assign to them. If you want them to be an Admin, select "Full Control." Click on the "Give Access" button to add the person to the page.

The person you have added will receive a notification and will need to accept the invitation to become an admin of the Facebook Business Page. 

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To add someone to a Facebook Business Page, you need to have admin privileges yourself. If you are already an admin, here are the steps to add another person:

How to Give Someone Access to Facebook Business Page