Google looks to be expanding the user base of Inbox. They’re giving each current Inbox users 10 invites each to share with friends and family. To share the love (or at least Inbox invites) to others, you just need to tap on the Compose icon at the bottom and select the golden ticket icon. There you can put in the email addresses of the people you want to invite. Remember, Inbox is strictly Gmail at the moment.
Source: Cult of Android