Sorry, my fault. It seems you can´t do that from iCal, you have to do the delegation in the terminal, on the server to be able to delegate your calendar to a group (what a pain in the ***. WHY APPLE???)
Anyway, in case you still want to try it, here´s how I did it.
for each user, use one of the following commands in the terminal on the computer running iCal server:
(replace <groupname> with the appropriate group and <username> with the users shortname)
*For read-only access (all on one line)*
sudo calendarserver
manageprincipals --add-read-proxy groups:<groupname> users:<username>
*For read-write access (all on one line)*
sudo calendarserver
manageprincipals --add-write-proxy groups:<groupname> users:<username>
Authenticate with your admin password.
Use a different group for read-write access members, or add specific users as proxies in the iCal client.
You can list all the proxies set for a user with the following command:
calendarserver
manageprincipals --list-proxies user:<username>
Everytime you create a new user, you have to apply this command to that users account, to allow the group to see his calendar.
After this you have to restart the iCal service on the server. If you don´t, you will probably get error messages in the iCal client (I did).
The users must then choose to show the other calendars in the preferences for their account in the iCal client (same place you delegate in iCal).
Hope this helps.
Why Apple couldn´t make a GUI for this is beyond me.