Google has been rolling Inbox invites out at a somewhat brisk pace, but if you find yourself still waiting, there's a surefire way to get one today. Just send an email to [email protected] between 3 and 4PM PT today and you'll have your invite by 5PM.
— Inbox by Gmail (@inboxbygmail) November 5, 2014
For those of you in other time zones, that's about 10 minutes after this post went live (5PM CT, 5PM ET). If you're reading this later, you missed the boat—sorry. The email doesn't have to say anything in particular, but you should get a reply with your invite by 5PM PT.
Google is calling this "#InboxHappyHour," but it's not great for people outside the US (the middle of the night is not suitable for happy hour). Google will reportedly do another invite giveaway that is timed more conveniently for other time zones, though.