There are ways providing shared access to user’s mailboxes on Office365. Unfortunately Office365/Outlook 2010 does not give you the same level of mailbox administration as you have with an on premises system.At times you just need to give the users a proper full mailbox access. To do that, you will need to do it from a Microsoft PowerShell.
Note: by default Microsoft PowerShell in order to protect the computer against malicious scripts won’t allow you to run unsigned scripts. To successfully run the commands in this article, you will need temporarily change the mode of the PowerShell. Follow my article Running Microsoft PowerShell in unrestricted mode to achieve this.
To start, you need to connect to the online service. To do so, start with the command:
> $LiveCred = Get-Credential
Running this command will present you with a username/password prompt. You will need to enter the Office365 administrator details here. Next you need to setup the session. Run the following 2 commands to achieve this:
> $Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://ps.outlook.com/powershell/ -Credential $LiveCred -Authentication Basic –AllowRedirection
> Import-PSSession $Session
Once the session is setup, you can manage your Office365 system using Exchange 2010 commandlets. To provide UserB full permission to UserA’s mailbox, run the command:
> Add-MailboxPermission -identity UserA@domain.com -user UserB@domain.com -AccessRights FullAccess