The armor will be given to anybody who pre-orders Halo 5 from any GameStop store in the United States. There hasn't been any details on how long players will have to pre-order to be eligible for the bonus but it is clear that you'll definitely need to order Halo 5 before it is released.
So far, no other retailers have given details on any pre-order bonuses, so GameStop is definitely looking like a good place to pick up Halo 5 so far. On top of the armor, players will be able to get a free poster for pre-ordering Halo 5 as well. Below, details about the bonus armor has been described.
HUNTER-Class Armor: Sophisticated and lethal, HUNTER-class armor incorporates Watershed Division's latest warfighting technology.
HUNTER-Class Helmet: Though still considered a prototype implementation, the HUNTER's sensor suite enhances situational and threat awareness.
The HUNTER-class armor is the same armor that Agent Locke wears, and it seems like players who do not pre-order will not be able to pick up the same armor. Even though the armor is most likely just cosmetic, I personally think it seems a bit unfair that some players won't be able to dress as Agent Locke in Halo 5, especially considering he will be playing a very important role in the story of Halo 5: Guardians.