The Night Guard is separate from the Crimson Guard's Shadow Guard. The Night Guard are elite Vipers that have chosen not to join the Crimson Guard as they desire a more hands-on active role vs. the more covert operations of the Shadow Guard.
They are all chosen for their skills and abilities. They have all achieved Expert or higher status in marksmanship. They can be found at all Cobra urban installations, and anywhere extra security is called for.