Summarize this article:

It's a white app with an orange flag icon. Facebook Page is available for free from the Apple App Store or the Google Play Store.  Choose the page to which you'd like to add an admin.  It's at the bottom of the menu in the "SETTINGS" section. It's near the top of the page. It's at the top of the screen.  If prompted, re-enter your password. You must be a Page admin to give someone else an admin role. If you're not already Facebook friends with the person you want to add in an admin role, send them a friend request and wait for them to accept before attempting to make them an admin.   The friend you added can now administer your Facebook Page when they log in to Facebook. Page admins can perform any function on the Page, including editing content, purchasing ads, and adding or deleting other users.

Summary:
Open the Facebook Page app. Tap ☰ in the upper-left corner. Tap a page. Tap … in the lower-right corner. Scroll down and tap  Page Settings. Tap Edit Page Roles. Tap  Add Person to Page. Enter the name of a Facebook friend. Tap the name of the person you want to add. Tap Admin. Tap Add.