They are a stationary Colonist. To assign a Colonist to be a guard, you must pick if you want a Nighttime or Daytime guard, then pick which type of guard. Guards can be attacked and quickly killed by Monsters at night.
The Slinger requires a Sling and a Sling Bullet to attack. The Bow Guard requires a Bow and a Bronze Arrow to attack. The Crossbow Guard requires a Crossbow and a Crossbow Bolt to attack. The Matchlock guard requires a Matchlock gun, a Gunpowder Pouch, and a Lead bullet to attack.You can assign Daytime or Nighttime guards, Nighttime guards will sleep during the day, and Daytime guards will sleep during the night.