Inviting some via private folder

Is a twitter handle the only way?

The docs say I can create one that's private/username,friend@twitter

Is there a way to also set one up private/username,friend@emailservice.com?

The issue names only the private-folder documentation and the private/username,friend@twitter syntax; no file, test, or entry point is provided. Start by checking that documentation and the current invitation behavior, then establish whether email-service recipients are supported and what the expected behavior should be.

Written by the indexing model from the issue text.