A good UI would make it explicitly clear by highlighting the email address and/or by auto adding an explicit entry to the guest list.

At present there's no way to predict the behavior until after the fact. Something with that much consequence (sending an email to an unintended recipient) should never be done silently. By all means take the initiative with autofill etc, but the user should have the final say.

If the supposed design revolution within Google (http://www.fastcodesign.com/3016268/google-the-redesign) isn't just for show, the Calendar team (assuming there even is an ongoing team) clearly hasn't been touched by it and that ought to be corrected.

When the user gets unexpected behaviour doing something that many believe to be reasonable, telling them they're doing it wrong is really bad user interaction.

You can't change the behaviour of users, but you can make your software easier to use and more predictable in its behaviour.

You know what, reading all your desperate posts is very disturbing. Even Google Now knows to ask before auto-doing shit, but you sit here with some stupid surgeon-like "I can do no wrong" attitude.

Generally, I thought Google hired smarter people than you seem to be, so possibly you're some sort of false-flag bullshit.