So if you're here you're trying to add someone new as a member of your Facebook Business manager.
When you add them they are sent an auto invite but sometimes this auto invite fails. In my case, ALWAYS.
So it turns out there's another secret way to just that person a link and its WAY easier than you might think!
Follow the usual steps to add the person as an admin to the page.
At this point you will be in users.
When you click resend you will see a link that you can copy & paste to the person you are needing to add.
And youre good to go!