Ok, your last post helped clarify this. In larger companies, Exchange server is the common store point for calendar entries. Then everyone, regardless of Windows or Macs, can see the same basic calendar data. Without Exchange, and if you are a small business that might have too much cost and complexity, unless you look into having some other company
host it for you, and just pay them a fee.
Barring that, you could look into sharing data using a public service like Google Calendars, (our band uses this method), Yahoo, MSN Hotmail, etc. My only concern would be if a business was doing work that was very secret. It might be too easy to somehow let your information out into the wild by a user mistake (some calendars could be made to be Public instead of private for example).
Other methods, like the PDA methods,are a bit of a hassle for a business. Depending on your budget, I'd look into hosting it internally to the company, externally, and also using a public service like Google. You can then weigh the pros and cons.
I understand that the next version of OSX should have some better server based calendaring. That might be worth waiting for, but I have no idea what features it has.
Group: Did I miss any other options?
Hope this helps.