LibCal Authentication
The calendar events within LibCal can be set to require authentication in order to register. The same is true for spaces that allow the user to make a reservation.That choice is made within the event in Libcal, however, the authentication, itself, is one level up LibApps > Admin > LibAuth Authentication
We are using MyAccess (Shibboleth) at UCSF for authentication.
An alternative way to limit registrations is by only accepting certain email domains. However, a user can fake an email address since there is no authentication against a lit of known users.