How to setup Calendar permissions for group to group

The answer is that you can't do a group to group permission setting.

However, you can add security groups to a calender. This is how I got it to work.

For example:

Create a Security group that is Universal (Cannot be Domain or local). Add users to this security group. In Exchange, add a new distribution group and select this new Security group as the target group. After the group has been enabled as an "email enabled security group" you can assign the permissions for the person's calender to the group using this command:

add-MailboxFolderPermission -Identity USERID:\Calendar -User Sercuritygroupname -AccessRights Owner

Make sure to hide this group from the Global Address List to prevent errant emails from going to this group