Joined Xbox Insider Delta but didn't got the features

To join just download the insider's app on your Xbox open it go to settings and join whichever ring you are able to. You may not be able to join beta ring until you have been a member for something like three months so you may have to join delta temporarily.

Seems you have to be invited to join the Alpha ring

https://support.xbox.com/en-US/xbox-one/console/xbox-insider-program

The current Xbox One Update Preview rings are:

• Alpha – Open on an “invite only” basis and receives updates at the earliest point in the preview window.
• Beta – Open to Xbox Insiders with a Program tenure of 3 or more months and an Xbox Insider XP Level of 5 or higher. This ring receives updates shortly after Alpha.
• Delta – Open to any Xbox Insider with a tenure of 1 or more months that has reached Xbox Insider Level 2 or higher. Depending on certain edge cases where further attention is required from Alpha or Beta rings, Delta Insiders may receive
  system updates before Beta, though this is not a common scenario. Additionally, while early system updates are not guaranteed across the Delta ring, all Delta Insiders will receive the update prior to GA.
• Omega – Open to anyone and receives system updates shortly before the GA (General Availability) release.

The decision to join the latest iteration of XBox Insiders, 18 months ago, was made to solve some issues I was having when I purchased
a XBox X and couldn't find any satisfactory feedback.

Joining gave me the resources to Report a problem using the Insider Hub which I have used many, many times initially
in the Omega, then Delta, and now Beta Rings.

The Reddit XBox Insiders Forum and the Reddit XBox
Insider wiki for all things Insider related is also a great resource to help with issues and to provide information on upcoming update releases to the XBox console operating system.

Deciding what ring to join depends on what level of involvement you want to have with the program.

The current Xbox One Update Preview rings are:

• Alpha Skip-Ahead – Open on an “invite only” basis and receives updates as part of a different future branch. This ring receives updates far ahead of General Availability (GA), with features that may not exist in the other rings.
• Alpha – Open on an “invite only” basis and receives updates at the earliest point in the preview window for the current development branch.
  
• Beta – Open to Xbox Insiders with a Program tenure of 3 or more months and an Xbox Insider XP Level of 5 or higher. This ring receives updates shortly after Alpha.
  
• Delta – Open to any Xbox Insider with a tenure of 1 or more months that has reached Xbox Insider Level 2 or higher. Depending on certain edge cases where further attention is required from Alpha or Beta rings, Delta Insiders may receive
  system updates before Beta, though this is not a common scenario. Additionally, while early system updates are not guaranteed across the Delta ring, all Delta Insiders will receive the update prior to GA.
  
• Omega – Open to anyone and receives system updates shortly before the GA release.

I'm still waiting for an invite but quite happy with Beta.

That feature is only available to Alpha,Alpha skip aheads & Beta Insiders at the moment.

As you are in the Delta ring you will still need gold to play any games online.

I'm currently in Delta, use to be in Beta but I didn't want to update as often as they do. I've been an insider for nearly 5 years now which is crazy.