I believe they'll add it in matchmaking sometime on October. You can currently play it in custom firefight games by playing on the Blue Team. Spartans are the Red Team and Elites are the Blue Team. I found this out by accident by starting a custom firefight game by myself while I was on the Blue Team. I spawned as an Elite and the Covenant came down on drop ships and just walked right past me standing still because there were no Spartans.
I betrayed two and the entire Covenant just unloaded me killing me in less then a second and continued to do so when I respawned.








