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