There is a special class of users, guest users, which are added directly by regular users when they give sync access (regardless of the read/write permissions granted) to external collaborators by providing their email addresses.
Guest users have access to a subset of the functionality of the Sync Appliance so as to minimize their impact on the system, yet allowing convenient ad-hoc collaboration.
Guest users:
As a consequence of these restrictions, guest users can only work with the data from other regular users and thus introduce essentially no additional space requirements.