To use Google Inbox service, you should have an invitation from Google based on your usage of Google services. But you can even ask Google for Invitation of Inbox, by sending a simple email requesting for this on email@example.com. You will get confirmation in reply from Google. They will say,
"Thanks for requesting an invite. We'll send you one as soon as possible. Your friends at Gmail"
Google Inbox Features:
It helps to organize the things better and keeps the stuff simple. 'Bundle' helps you to keep your similar stuffs and messages at one place. You do not need to look into the large dump. 'Highlights' features highlights the important instructions, attributes and information without even reading or opening the email, thus saves time. 'Reminders' features helps to remind the important events, happenings and meetings by built in support. 'Snooze' helps you to remind again the important message so, you cannot miss the deal/event/happening and also you can set time for next appearance as fresh email. 'Search' helps you to find what you exactly need based on the email content. 'Works with Gmail' feature keep the mail from Gmail at one place and will become bridge between Inbox and Gmail.