Growing the Circle is not restricted to whoever created it. That is deliberate: the person who thinks of inviting an aunt is often not the person who set the account up.
Who can invite
The Thriver, the older adult herself.
A Family Advocate.
Any family member in the Circle.
Who cannot
A professional caregiver. They can do their work in the Circle, but they cannot add people to a family. That boundary is fixed and not a setting.
Inviting and paying are separate
Anyone who can invite can invite. But if the new person is past what the plan covers, the invitation does not add a cost on its own: the person whose card is on file is asked to approve the seat first.
So a sister can invite your cousin without being able to raise your bill. See Someone accepted my invitation and now I am being asked to approve them.