The Inbox by Gmail team is promising that if you send an email to: email@example.com between 3-4pm PT you will receive an invitation to join by 5pm.
The Official Tweet:
Want an invite TODAY? #InboxHappyHour 3-4pm PT. Send us an email at firstname.lastname@example.org between 3 and 4pm PT and you’ll get your invite by 5pm
— Inbox by Gmail (@inboxbygmail) November 5, 2014