You can add Calendar Permissions Using Powershell for users using the following command:
if user2 needs to access user1’s calendar
Add-MailboxFolderPermission -identity “user1:\calendar” –user “user2” -AccessRights Reviewer
the same can be done to give permission to a security group
Add-MailboxFolderPermission -identity “user1:\calendar” –user “DomainName\securitygroup” -AccessRights Reviewer
Below is the list of access rights and explanation
None FolderVisible
Owner CreateItems, ReadItems, CreateSubfolders, FolderOwner, FolderContact, FolderVisible, EditOwnedItems, EditAllItems, DeleteOwnedItems, DeleteAllItems
PublishingEditor CreateItems, ReadItems, CreateSubfolders, FolderVisible, EditOwnedItems, EditAllItems, DeleteOwnedItems, DeleteAllItems
Editor CreateItems, ReadItems, FolderVisible, EditOwnedItems, EditAllItems, DeleteOwnedItems, DeleteAllItems
PublishingAuthor CreateItems, ReadItems, CreateSubfolders, FolderVisible, EditOwnedItems, DeleteOwnedItems
Author CreateItems, ReadItems, FolderVisible, EditOwnedItems, DeleteOwnedItems
NonEditingAuthor CreateItems, ReadItems, FolderVisible
Reviewer ReadItems, FolderVisible
Contributor CreateItems, FolderVisible
Exchange KB 
Good post, you are missing one important item on your list of access rights “limiteddetails” which allows free/busy subject and location. also you do not need the domain when adding a security group.
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Does not work… you have to add distribution group in exchange management.
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you can’t add a security group, it’s only for users that already have an email account on the server (it’s the equivalent of calendar’s tab “access rights” in outlook).
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