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.
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.