Hello jwrw
You can in usergroup specify credetials for each category ( or all ). You can specify if users are allowed to create/add events and/or allowed to view events.
But this is of course dependent on, if you are logged into the calendar or not.
If you use the displaym you can have a display that shows some categories from the calendar and this display could be set up to be shown on the public pages. Then you could have a copy of the same display to be set up for the logged in area, and in this display could be added some more catagories to be displayed.
You can also have some selections set on the calendar in the public area ( via parameters ) and some other parameters on the calendar set up in the logged-in area. ( the parameters for the calendar will be as if you initially had selected in the options-button). You should therefore maybe remove the options for the public calendar.