Add/Give Calendar Rights

Add-MailboxFolderPermission -Identity user1@xyz.com:\calendar -user user2@xyz.com -AccessRights Editor

Add-MailboxFolderPermission -Identity user1@xyz.com:\calendar -user user2@xyz.com -AccessRights Owner

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Delegate - 

Set-Mailbox user1@xyz.com -GrantSendOnBehalfTo  user2@xyz.com

Add-MailboxFolderPermission user1@xyz.com:\calendar -User user2@xyz.com -AccessRights Editor -SharingPermissionFlags Delegate