You can add Calendar Permissions Using Powershell for users using the following command

1#
$UserCredential = Get-Credential

2#
$Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://outlook.office365.com/powershell-liveid/ -Credential $UserCredential -Authentication Basic –AllowRedirection

3#
Import-PSSession $Session

4#
Get-MailboxFolderPermission user1.name:\calendar

#5
Add-MailboxFolderPermission user1.name:\calendar –user user2.name –accessrights reviewer
(ie above gives user2 reviewer rights to user1 calendar )

#6
Set-MailboxFolderPermission user1.name:\calendar –user user2.name –accessrights editor
(Replacing user.name with the user’s usernames)

#7
Remove-MailboxFolderPermission user1.name:\calendar –user user2.name

#8
Remove-PSSession $Session

Send on behalf of etc

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

Add-RecipientPermission user1@domain.com -AccessRights SendAs -Trustee user2@domain.com