For now, calendar sharing is Zimbra user to Zimbra user. But we will be working on ways to publish calendars to Zimbra controlled domains, externally, etc. Unfortunately, I don't have a timeframe for you at this time.
In the short term, you can export any of your calendars as an .ics file and post that to some public web-accessible location (e.g. corporate wiki, etc). Then the users can create a calendar and sync to the posted .ics file.
Here's an example to get your default calendar:
http://hostname/zimbra/user/~username/
calendar.ics
The part in italics is your username used for logging in and the part in bold is the name of the calendar
folder. If you created another calendar called "personal", then the URL would be:
http://hostname/zimbra/user/~username/personal.ics
Here's some useful links:
Publishing/Viewing Calendar from Sunbird http://www.zimbra.com/blog/archives/...st_and_no.html