When creating a new circle you can choose whether you want to be added as managing user in this circle or not. If you intend to add another user as circle manager after creating the circle, you can deselect this option.
As a Site Administrator you will still be able to access a circle, which you are not explicitly a user in, including editing users. But you have to be a managing user to create stories etc in the circle. You can at anytime add yourself as a user to any circle in you site.