Note: A page has to join the group first to make the page an admin
Once you are in your Facebook group go ahead and click People.
Once you are in the People section, find the Facebook page where you want to make an admin and click the little three dots as shown in the picture.
Once you click that you should see an option saying Invite as admin
Go ahead and confirm it.
After that a person who received an invite should go to the group and see a message saying that a person invites you to be an admin of this group, go ahead and click Accept.
Once that is clicked you should be joined as an admin to the Facebook Group.