The old and easy way of adding an admin to your facebook page (assigning page role) has been replaced with assigning ‘page access’ instead:

Open up the facebook business page

Click Settings (gearwheel left menu) > New Pages experience > Page access

To assign a new admin to your Business Manager account (the overall owner of the meta assets – i.e. pages, commerce, insta etc)

Open up Meta Business Suite:

business.facebook.com > Click Settings (bottom left menu) > People > Click the ‘Add People’ button and enter the persons email address in the box.

Assign the appropriate permissions and facebook will send over an invite to be included.