I don't see any place to add a calendar within the Google Group. I
believe that someone would have to create a Google Calendar, then
share it with the group email address. Everyone could then see it, but
I don't think you could embed it in the Google Group website or
anything like that. They'd have to have the URL and go to it. If you
already use Google Calendar, it would be super easy, but for those
that don't, it wouldn't be too convenient.
I could easily be wrong, but that's what I found after a little poking
around. Google is behind Yahoo in this regard, I think...
-T