Here are the instructions to grant access to a shared mailbox in Office 365 / Exchange without automatically mapping it in Outlook. This can be useful if you require access to the mailbox on occasions via web browser but don’t require it to be showing in Outlook.
Connect to Office 365 using Powershell. The full instructions can be found here
Run the following command
Add-MailboxPermission -Identity email@example.com -User firstname.lastname@example.org -AccessRights FullAccess -AutoMapping $false
You will be presented with the following confirmation:
Wait. Sometimes this can take up to 1/2hr for the changes to take effect.