Hi @cramer,
I’ve looked into this a bit and have some more information.
The guide on invitation flow linked above assumes you will be using database connections only (not social connections such as Gmail/Google).
What you could do is follow the guide and create users in advance as database connections. You could then use the Account Link Extension to merge their accounts if they choose to log in with Google at any point.
Related Community topic: How to disable sign up via social login? - #5 by vacilando