I just tested this and found what I was expecting. When enable local logins is disabled, clicking the invite link displays a “Sign in with Google” modal:
When enable local logins is enabled, the username/password modal is shown:
The text on the signup modal lets users know that they’ll be able to login with any social logins using their email address. I think users will understand this to mean that they’ll be able to sign in with Google the next time they login to the site.
With the username/password signup, users are logged directly into the site when they click the “Accept Invitation” button, so the only friction I can see is that they are forced to enter a password before they can initially access the site.
I don’t think there is any way to get the “Sign in with Google” button to display on the invite modal if local logins are enabled.