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, at a minimum, the meeting or meeting poll organizer needs to be a Registered User. Other meeting participants can be Ad-hoc Users and provide their availability manually by voting on proposed times. 

However, if meeting participants are Registered Users (which implies they connected their calendar to FreeBusy and are sharing calendar availability) their availability can be checked in real time. So, the more meeting participants who are Registered Users the shorter the time to reach consensus about the best meeting time. 

If all participants to meeting poll are Registered User the poll can be avoided all-together because the organizer can see everyone's calendar availability and find the best time to meet instantly.

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