Welcome to the forums,
Sharing calendars is easy (you right click the calendar and select who to share with)
You can share with:
Internal Users-anyone in your domain with a zimbra account (let them manage or view only) the calendar shows up in their list once they accept the share.
External Guests-any other email address, just assign them a password (view only)
Public (view only)
I'd leave 'send a message' as is (or 'send standard message with note') to let them know about the share.
You can always resend the message if they don't accept the first time and loose the 'do you accept this share?' email.
Or direct them to the URL.
More info on the calendar:
http://www.zimbra.com/products/zimbra_calendar.html
You can also currently share address books and document notebooks. (official screenshots
here)
Also coming in v5:
-I know you said "so a secretary could not view email" but just letting you know that shared mail folders will be here soon. So an exec could share just one folder to the secretaries so they could respond/follow up to certain emails.
-Tasks (sharing possible too of course)
-Chat/Instant Messaging
(check out the beta)
You will also be interested in the Groupwise migration wizard...makes your job a lot easier.
The only issue with it that I've had is importing existing 'meeting rooms/resources'. I just ended up making new locations/resources for simplicity. (there we're only like 5 rooms for this particular setup).
Probably would have just taken a few minutes to solve if I had actually cared-I just found out who managed them, made new ones,and told them to phase out the old stuff on their calendars. Simply open a support ticket for if you have trouble.