Add Shared / Delegate Mailboxes to Outlook mobile app
- Tap on either your picture or initials in the top left corner of the Outlook app
- Tap the settings gear icon in the lower left corner
- Tap "Add Mail Account"
- Tap "Add Shared Mailbox"
- Enter the email address you have access to, and tap "Add Shared Mailbox"
Attached PDF has photos of each step