Universal but secure calendar?

Hello. I am trying to find a software (prefer free since I work for the City and they don't pay me. hehehe) that will allow five law firms to be able to view the same calendar. The issue is that not all five firms use the same calendaring software. Two use Outlook while two others use "groupware". My boss needs to create an online calendar where all parties can view and if need be submit dates into a group calendar. Here's the kicker, it needs to be secure as this is a major case and he doesn't want someone to just walk off the internet and grab, manipulate, destroy the calendar. Any help would be greatly appreciated. Thanks!

 

There is Chandler, but I haven't worked with it personally however, and can't say more than that. There is some info about security here.

There is also Google Calendar.