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