Membership is not mutually exclusive with other organizations. That is, a member may also be part of the SCA (knight or not), other orders of Chivalry, LARP groups, etc. What they do outside of the Order is their own business, so long as it does not reflect badly on the Order or come in conflict with the Order's ideals or the ideals of Chivalry.
The Order itself is designed to be relatively low-key and informal in its structure. Member knights are all equal to one another, much like the Round Table of King Arthur. There are several introductory ranks before being squired and knighted as a full member. The "Commander" position exists as an organizational office, and is an elected position from within the body of knights.
The "Knight Commander" (or just "Commander") is the central organizer for the Order. This does not mean that he or she is solely responsible for events, activities, and decisions. Quite the contrary, all member knights are encouraged to take up roles or activities, and major decisions are by vote. However, since a central authority is sometimes needed, this office exists.