Registered Users are individuals who registered for a FreeBusy account and who use FreeBusy to:
share calendar availability, schedule meetings and participate in meeting polls, or
share calendar availability, schedule meetings and participate in meeting polls on behalf of another Registered User, acting as a delegate of that other person.
Ad-hoc Users are individuals who use FreeBusy to respond to meeting polls organized by Registered Users and/or to propose meetings with Registered Users via FreeBusy links and who are not Registered Users themselves.
Pricing is based on the number of Registered Users you are purchasing a subscription for. When you purchase a subscription you purchase a set of licenses you can assign and reassign to Registered Users while your subscription is on-going. Registered Users in your subscription can be from inside and outside your organization, whether they share the same email domain or not.
Ad-hoc Users do not need to register for an account with FreeBusy and they do not need a license.
When scheduling meetings with FreeBusy, the organizer and participants can see calendar availability in real time only for Registered Users because only Registered Users have their calendar connected to FreeBusy and can share calendar availability. Ad-hoc Users will have to provide their availability manually by voting on proposed times in meeting polls or by proposing new times to scheduled meetings.
So, while it's not required for all meeting participants to be Registered Users, it's beneficial if some or all participants become Registered Users so you can check their calendar availability in real time and find a time that works instantly without having to wait on people to respond to meeting polls.