Previously, Xbox One Insiders could opt in to the Insider Program, and were then sorted into one of four tiers, the last two called "Rings." Each group had different update schedules for new Xbox and game changes. Instead of Rings, Microsoft has renamed the last two levels to be more in line with the original first two tiers. Now Insiders will separate into Alpha, Beta, Delta or Omega groups. The new system also allows Insiders to more easily move between groups as their needs and bandwidth caps dictate.
Alpha is an invite-only tier reserved for Xbox Insiders that provide the most customer feedback and are more active on their console, while Beta is open to members who have been in the program three or more months with an higher Insider XP level. The Delta group will let anyone with a tenure of at least one month, while the Omega group is for pretty much anyone interested. The Alpha tier will get many more updates and earlier access than the Beta group, who will have earlier access and more updates than the Delta group, and so on.
Signing up to become an Xbox One Insider is pretty easy, too. You can hop onto your console, open up My Games & Apps, then select the Xbox Preview Dashboard from the Updates tab. Once downloaded, the app will rename itself to Xbox Insider Hub, where you can access all the Insider perks.