The invitations are appearing between pages’ admin panels and cover images, and they read:
Coming soon: a new layout for Facebook pages. Add your page to the wait list to try it out.
Page admins who join the wait list then see the following message:
Your page will be updated soon. It may take a few days for the new version of your page to be updated for other people. We’ll let you know when the new version is ready.
Page admins: Have you seen these invites?