Only people who receive an invitation can use Inbox by Gmail, and they can only access Inbox through their personal Gmail account, not through their Google Apps accounts. People who receive an invite to their Google Apps account can forward it and use it on their personal account if they want to.
https://support.google.com/a/answer/6082718
Got the invite for my google apps account, tried to sign in and was denied - because it was a google apps account.