if they use outlook, they can set the internet free/busy url to point to the zimbra server.
In outlook:
1) Go to:
tools->options->calendar options->free/busy options
2) Set the search location to:
http(s)://<your-server>/service/pubcal/freebusy.ifb?acct=%name%@%server%
when you attempt to create a meeting request with
user@company.com, outlook will send a free/busy request to
http(s)://<your-server>/service/pubcal/freebusy.ifb?acct=user@company.com