Hi Keith,
The “add to circle” function means that the person become a circle member of the circle. If you want to add new persons as circle members to a circle, you need to ask the Lead Link of such circle to do it because only the LL can say that someone is a circle member even though this person doesn't fill any role.
Please find a screenshot to see how to add someone as circle member: https://drive.google.com/file/...U3M/view?usp=sharing
As you will see, a little red star will appear if the person doesn't have any role within the circle.
Hope that helps!