Queen's Guard
Traditionally, the Queen's Guard is made up of unbelted fighters in the kingdom. This tradition is a holdover from when we were part of the West. Usually, it is the best fighters among the unbelted ranks but can also include fighters that the Queen believes are deserving of the honor even though they might not be the top rank of unbelted fighters. In the West, the Queens Guard used to have to meet a group of knights in a melee at least once during each reign. Usually the Queen will ask a knight to preside as a Knight Councillor over the guard. This is separate from the Captain who is usually an unbelted that that been on more than one retinue.
The Guard is made up of whoever the Crown would like. Generally the Captain(s) is a knight (or darn close to it) and the Guard itself is made up of fighters, any combination of heavy, archer and rapier, but even that is more of a guideline and not consistent. It all depends on the whim of the Crown.
Ula
Note that some crowns or others have make more general populace calls for guard duty, with no stipulations at all other than a willingness to serve and hold a spear or other symbolic item. This is an excellent way to get introduced into what guard/retinue duty is like.
See also: Retinue