Circles are completely controlled by the users. The event owners will likely create the primary circles, and then users also have the ability to create circles, which can be private (no one will even see them in the list) or seen by all other collaborators.
Create a Circle
1. Click the circle with the plus inside.
2. Type a Circle name (as brief as possible).
3. Select a color for the circle.
4. Click the private button* only if you want it to be seen exclusively by you.
5. Click Create.
The new circle will display in your event, and you will be the only member of it until you add in other collaborators.
* The lavender circle with the lock on it above indicates a circle that is private -- budgets tend to be a good option for this capability. You CAN invite others into it, and no one else will see it.