Microsoft brought out the big guns for the beginning of their E3 conference today, with 4 player co-op footage from Halo 5: Guardians. Halo 5 features drop in/out co-op as players control either Commander Locke and his team of Spartan hunters, or the Master Chief with a crew of his own.
Why is the Chief being hunted? We’re still not entirely certain but during the demo they mention him being absent without leave and hiding in the last covenant stronghold.
https://youtu.be/yx27jORxGhs
They also took the time to show a bit of multiplayer footage. They revealed that player counts will be as high as 24, and that the maps are up to four times the size of previous Halo maps, and can even include AI players.
Source: YouTube