Microsoft has announced that it will deploy the ability to add shared mailboxes to Outlook Mobile by the end of July. But if you want to see the feature early, you can join the Testflight program and install the beta version of Outlook mobile. Using Outlook for iOS with Testflight also forces the upgrade of your Office 365 tenant to the Microsoft Sync Technology.
Microsoft has announced that Outlook Mobile (iOS and Android) will include support for Exchange Online shared mailboxes “in the next several weeks,” which probably means early July 2019. The update comes as good news for many people who have been forced to use an IMAP4-based workaround to access shared mailboxes. Microsoft is also making some other changes to improve the Files view and calendar sync in Outlook mobile.