That information is not available. You can get the list of members,
but not invitees. This is an opt-in scenario.
Yeah, invitees should have the option to stay anon. I would not like it if someone invited me and all of a sudden, that site, which I didn't care for, had my Google account and email west, etc.
That would be a giant no no.
There are no API calls that will allow you to sign someone up for GFC.
Again, this is an issue of opt-in — if you could sign someone up
through an API, you could sign them up without their knowledge.