Microsoft has rebranded its Xbox Preview Program as the Xbox Insider Program and opened it up to everyone.
Previously you needed an invite to enter the Xbox Preview Program (XPR from here out) with those invited getting early access to games, apps and Xbox system updates. Under the new Xbox Insider Program (XIP) users will just need to opt in to access the experimental games and apps coming to the device, although the system updates will be limited to those that already have access to testing them, with Microsoft claiming they already have a large number of volunteers already providing feedback on the system updates.
The XIP will be supported by its own selection of the console, the Xbox Insider Hub, where you'll be able to access the new features in addition to seeing announcements and opportunities for new content regularly.
These changes are rolling out slowly with a small percentage of players being given access now. Microsoft have said that over the next few weeks, all XPP members will be switched over to the XIP and given access to the Xbox Insider Hub.