Not from the user UI at the moment, unless you have a distribution list (aka group) will all members of the domain in it.
Zmmailbox - Zimbra :: Wiki
So you'd start with:
zmmailbox -z -m
share@domain.com mfg /Calendar domain domain.com r
That would give everyone permission but not mount the item.
To see it they'd need to right click the word 'calendars' just under 'new calendar' > select 'link to shared calendar' > enter the info.
Or you could provision with:
zmmailbox -z -m
user@domain.com cm --view appointment /SharedCal
share@domain.com /Calendar