I share my calendar with many people. I don't want everyone to be able to edit the calendar.

I share my calendar with many people. I don't want everyone to be able to edit the calendar. Every time I try to change someone's privileges to edit only, it always changes back to where they can view and edit the calendar. Can anyone give me suggestions to how to allow then to only be able to view it? I can't make it a public calendar because it is work events we don't want the whole world knowing about.