As of 12/7/2023, Office 365 allows you to use two different versions of Outlook. The Old, and the New.

The new version has an updated UI, they've moved around some things, and generally just did an update of how Outlook runs.

In the Old Outlook, you could see your shared mailboxes right along with your personal email address, which was nifty and convenient.

Regrettably, they took away this option in the New Outlook, you must now open Outlook, navigate to the "Shared with me" drop-down menu, and find your groups from there.

(Note: as of right now there is no way to make the mailbox appear along with your own, UNLESS you statically set and add the account via it's credentials.)