By Preview they meant if a person being part of the Preview Program can invite him to that program.
Clubs are currently tested in Preview.
Preview Program is just a testing program you can volunteer to be part of. It has pros and cons - your console will update 1-2 times a week with mostly 450mb updates, there can always go something wrong with the new update leaving you with some stuff not working, but you get to test new things out first and use them.
When Backwards Compatibility launched on XOne there have been massive requests of people willing to join just to be able to play those games.