Members of your organization can be added to a circle in two ways.

One way is to assign them to a role in a circle.  See the following article for help with that:  How do I assign someone to a role?

The second way is to add them as a member of the circle without a role assignment. Go to the circle page and click the "Members" tab.  Then use the Add Person button to make the assignment.  (The button is available to the Lead Link of the circle, the Secretary of the circle, and your organization's Admins.)

Related: How do I add a new user to my GlassFrog organization?