Snow Leopard's iCal Server changed the location of the Wiki-based calendars, so that this previous hint no longer works. Intead of using the path of /principals/groups/groupname, the URL for wiki-based calendars has changed to one of the following:
https://ical.domain.com:8443/principals/wikis/groupname/
http://ical.domain.com:8008/principals/wikis/groupname/
When you add this in as a URL for an additional account, you can then access this shared calendar, with your changes reflected on the wiki page. You could alternatively create a specific user in workgroup manager to be used as the 'silent' owner of a group calendar, create your calendars, assign delegation to all the real users. But if done in that way, you don't get to see the calendar in the wiki, online.
Mac OS X Hints
http://hints.macworld.com/article.php?story=2009090507094085