I noticed the below exchange about two people accessing the same calendar. I take the chance, but only add items to my colleague's calendar when I know he is away. Accidentally I suppose I could enter an item in his calendar at the same time he did (forgetting to set the right calendar).
However, if a calendar could be set to Read-Only in the Calendar dialogue, it would be no problem me seeing my colleague's calendar, or our communal birthday/holiday calendar. (Yes, I can do it with access rights in the OS as well, but then it is trickier to gain access on an ad-hoc basis, when my colleague is away.)
So - not very high pri suggestion: Read only attribute to calendar.
Re:multiple users accessing local ics
I don't recommend putting the ics file to a shared folder if there is a chance that multiple clients access it at the same time. The system hasn't been designed to work that way so it is possible that the ics file gets corrupted if it is accessed simultaneously.