Hi there, David!
Interesting question. It is certainly possible to hide categories from the calendar (see this tutorial). A deeper customization might be to add a condition in there that checks to see if a specific visitor is logged in and is an administrator, then either hides or displays those categories.
You could also mark any event as “Private” or “Password Protected” which will require visitors to either be logged in or have a password to see a specific event.
Does this help answer your question? Please let me know. 🙂
Cheers!
Geoff