A Premium subscription user can create an Organization from their profile and pay for multiple users. The user can then manage the users within this same area (add/invite, remove, etc). All users added will be at the Premium level.
You can cancel your subscription at any time. A yearly subscription is paid in full upfront and is active for the year. If you cancel the service after upgrading, no refund is possible. (A renewing annual subscription can be refunded within 7 days of the renewing transaction)