I make a rule in each users mailbox when I create them called calendar. On that rule I tell it to automatically accept all appointments as free time from the user admin. We really do not use the admin GroupWise account for anything, but you could have it be any user.
Then, I go through all the firm holidays each year and make an appointment for each holiday needed and invite "everyone". Works like a charm. I also use this for meeting that are firm-wide, etc.