How can I access a Groups mailbox from the Outlook app?

From the mail view, tap the account (3 lines) icon at the top left corner of the app.

1. Tap the drop-down arrows to the right of the Microsoft 365 email account which has groups enabled to expand a list of folders for that account.

2. Tap on Groups to open it in the mail view.
   

3. Choose the group that has the shared mailbox you want to see.