I just received a Google Calendar event invite. When I clicked on “More event details”, the Google Calendar site launched in a browser window. But even though I was logged in to the site already, Google Calendar didn’t add the event to my own calendar once I’d confirmed. Odd. So I saved the “invite.ics” file attached to the mail, and imported the file to my own calendar via Settings -> Import Calendar. Now I have the event, but it’s a copy. There’s probably a better way to do this.