This article will show you how to add a shared mailbox to Outlook 365 Desktop or Web. The mailbox must be shared with the user prior to adding it to Outlook.
Outlook desktop client
The steps and images below are using the Outlook for Office 365 application on a Windows PC.
In Outlook desktop, in the left-hand folder navigation bar, right-click the name of the account (ex. flast@ssdmo.org) and select Folder Permissions.
Click Add
Click the drop-down menu and select the Global Address List.
In the search box, type in the last name of the person you want to delegate permissions to. The search box does search last names by default.
Locate the person you want to delegate permissions to and select their name.
Click Add.
Customize the permission level according to the access that the other person needs.