Like the invite system that is currently being used to allow new users to download the Inbox email app, Google is today allowing Google Apps for Work admins to request service access for their users. Google said it will let in only select users that request access as it works to get feedback from early adopters and improve the experience:
Best iPhone, iPad, & Apple TV game controllers
To start, we plan to work very closely with the early adopter companies, so not everyone that applies will be accepted right away, but the program will continue to expand over the coming months… Inbox wasn’t created to reinvent email, Inbox was created to help you reinvent the way you get things done. This means we need to understand more about how things get done (or don’t) today. And with your feedback, who knows, we could reinvent the way people work.
To request access, admins will have to email email@example.com through their Gmail Apps account. Google plans to start sending invites and open access for Apps users early next month.