This is actually a hint for both OS X Server 10.5 and iCal. I was trying to implement the calendaring feature in OS X Server and hit a wall when Apple's documentation did not explain how to set up multiple calendars for a single group, and how to manage them in iCal. After much searching, I found the answer (not supported by Apple) in this Apple Discussions thread.
In order to create a group calendar, you have create a group and enable calendars for that group. This is reasonably well explained in the "iCal Service administration guide for Version 10.5 Leopard" guide. In order to view the group calendars, open iCal (it has to be iCal 3), open its Preferences, and click on the Accounts tab.
There you will create an account by hitting the "+" sign and entering the following:
http://serverfg.private:8008/principals/__uids__/USERID/
In the above, USERID is a long string of letters and numbers corresponding to the path where the user's private calendar is stored. In order to add a group calendar you have to add the group URL instead:
http://servername:8008/principals/groups/groupname/
Replace servername with the host name of your server, and groupname with the name of the group in question.Mac OS X Hints
http://hints.macworld.com/article.php?story=20080410162942908