Sharing Outlook Calendar with Read/Write Access

If a user needs read/write or delegate or editor access to the calendar of another user within the organization, this can be done via Outlook App and PowerShell as well.

1. Share an Outlook calendar via Outlook Application

Ask User1 to share the calendar with User2 as per the instructions in the article below,

Share an Outlook calendar with other people - Office Support (microsoft.com)

Note: The permission should be "Can edit" or "Delegate"



2. Share an Outlook calendar via the PowerShell

Note: You can refer to Connect-ExchangeOnline (ExchangePowerShell) | Microsoft Docs

To allow User2 editor permissions over User1 mailbox calendar, please refer to cmdlet below,

Add-MailboxFolderPermission -Identity [e-mail]:\Calendar -User [e-mail] -AccessRights Editor

To check if the cmdlet results,

Get-MailboxFolderPermission Identity [e-mail]:\Calendar -User [e-mail]

For reference:
Add-MailboxFolderPermission (ExchangePowerShell) | Microsoft Docs
Get-MailboxFolderPermission (ExchangePowerShell) | Microsoft Docs