Please make sure you have installed Microsoft Online Services Sign-In Assistant for IT Professionals RTW, which can be downloaded here.

Open Powershell as an Administrator on your machine
Run the below cmdlets
Install-Module -Name AzureAD
$UserCredential = Get-Credential
A Windows Powershell credential request pop up will appear
Use your o365-admin credentials
$Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://outlook.office365.com/powershell-liveid/ -Credential $UserCredential -Authentication Basic -AllowRedirection
Import-PSSession $Session -DisableNameChecking

Use the below cmdlet to view a user’s calendar permissions;

Get-MailboxFolderPermission support@msbsolutions.com.au:\calendar

Use the below cmdlet to add calendar permissions to a user;

Add-MailboxFolderPermission -Identity support@msbsolutions.com.au:\calendar -user email@msbsolutions.com.au -AccessRights PublishingAuthor

Use the below cmdlet to remove calendar permissions from a user;

Remove-MailboxFolderPermission -Identity support@msbsolutions.com.au:\calendar -user email@msbsolutions.com.au

The following individual permissions are available:

CreateItems: The user can create items in the specified folder.
CreateSubfolders: The user can create subfolders in the specified folder.
DeleteAllItems: The user can delete all items in the specified folder.
DeleteOwnedItems: The user can only delete items that they created from the specified folder.
EditAllItems: The user can edit all items in the specified folder.
EditOwnedItems: The user can only edit items that they created in the specified folder.
FolderContact: The user is the contact for the specified public folder.
FolderOwner: The user is the owner of the specified folder. The user can view the folder, move the move the folder, and create subfolders. The user can’t read items, edit items, delete items, or create items.
FolderVisible: The user can view the specified folder, but can’t read or edit items within the specified public folder.
ReadItems: The user can read items within the specified folder.
The roles that are available, along with the permissions that they assign, are described in the following list:
Author:CreateItems, DeleteOwnedItems, EditOwnedItems, FolderVisible, ReadItems
Contributor:CreateItems, FolderVisible
Editor:CreateItems, DeleteAllItems, DeleteOwnedItems, EditAllItems, EditOwnedItems, FolderVisible, ReadItems
None:FolderVisible
NonEditingAuthor:CreateItems, FolderVisible, ReadItems
Owner:CreateItems, CreateSubfolders, DeleteAllItems, DeleteOwnedItems, EditAllItems, EditOwnedItems, FolderContact, FolderOwner, FolderVisible, ReadItems
PublishingEditor:CreateItems, CreateSubfolders, DeleteAllItems, DeleteOwnedItems, EditAllItems, EditOwnedItems, FolderVisible, ReadItems
PublishingAuthor:CreateItems, CreateSubfolders, DeleteOwnedItems, EditOwnedItems, FolderVisible, ReadItems
Reviewer:FolderVisible, ReadItems

The following roles apply specifically to calendar folders:

AvailabilityOnly: View only availability data
LimitedDetails: View availability data with subject and location

Please contact us here if you need any Technical Support.