Google Inbox Invites will be available for anyone for 24 hours. Google announced #InboxHappyHour for the next 24 hours and anyone who wants an Inbox Invite can send an email to email@example.com from their Gmail account to get a free inbox invite in minutes. Google Inbox App is still in beta stage and it has material design and works well for Gmail only users. Inbox App is still limited to Gmail accounts and Google Apps Email users are not allowed to receive an invite yet.
There is no news when Google will enable Google Apps for Inbox users, and that restricts many users to use it as their primary email client on iPhone and Android. The default mail app on iPhone, Android and any third party email service providers bring Unified Inbox feature and allows users to sign in with Gmail, Yahoo, Outlook, IMAP and POP accounts without any problem. That makes me to say Google Inbox is half baked and now they opened invites for public to gain more users as the Redesigned Microsoft Outlook Mail App is getting installed by iPhone users world wide.