hi..no built in function to do that other than mass email..but if you users alredy know where to go (i.e: they know zimbra login page) and assuming they all use WEB MAIL.
then the following customization can help
[SOLVED] AJAX Ticker On Login Page
by doing that you can show users the link to your how to / information about zimbra.
* what version of zimbra are you useing OSS or NE as in NE i guess there is something called HOME portal which users can see when logged in
Raj