4 Unlockable Armor Sets for Halo 5: Guardians Multiplayer Beta were revealed. You can unlock these by earning achievements in Halo: The Master Chief Collection

Nightfall
Watch the Halo: Nightfall Episode 1 Second Story (Unlocks Helmet)
Watch all 5 episodes of Halo: Nightfall (Unlocks Armor)

Mark VI
Earn the “Your Journey Begins” Achievement in MCC
Complete 1 match or mission

Mark VI Scarred
Earn the “Cairo Station” Achievement in MCC
Play and beat the Cairo Station mission in Halo 2 Anniversary

Helioskrill
Earn the “Legend” Achievement in MCC by completing all 4 campaigns on Legendary

Source: IGN