A Circle is essentially a grouping of Clients and Prospects. Using Circles makes bulk assigning resources like Portal Pages and Files to multiple Clients/Prospects easier, as you can assign resources directly to a Circle, and any users inside that Circle will automatically gain access to any of those resources automatically. You can manage Circles from the menu CRM-->Circles

From this page, you can create a new Circle by clicking the + button

This will open a form at the top of the page, with a few fields to fill out, including setting a Circle Name, Description, assigning desired Clients/Prospects, and setting a corresponding color.