Royal Guardsmen assist the Captain of the Guard in his duties of keeping the Monarch and others they are assigned to guard safe in feast or court. The guard is also sometimes assigned Security Duty at events, and may be expected to travel with members of the monarchy to various chapters during their reign.
How do you get to be one
Guards are either appointed by the monarch or by the Captain. So know that person and be willing to help them, and you might get in. Some times people are chosen for the guard purely because they are popular, or have a known name.
What do you get?
To look pretty, and maybe do some work. There are no titles or awards associated with just being on the guard. Guardsmen or women might receive service orders if they perform exceptionally. Guards are often given a nice tunic, favor, or sash to wear during events to identify them as guards.