This launches today for everyone.
From the Xbox thread:
And to that, I want to add Halo 5 now. On the last free preview week for Halo 5, I already finished the campaign, but after reading that long article on the history of Halo, I now have a desire to at least try out the multiplayer in Halo 5, especially Warzone. But even the base multiplayer is something I’d be interested in diving back into after reading about the various Halo games again. It brought back a lot of great memories of playing Halo games in multiplayer, especially Halo 1 and 2.