Outlook mobile app supports access to Microsoft 365 Groups when they are associated with a work or school Microsoft 365 subscription that has Exchange Online and SharePoint Online. That includes the Business Essentials and Business Premium plans, and the Enterprise E1, E3 and E5 plans.
Outlook for mobile does not support Groups in Outlook associated with free accounts or personal Microsoft 365 subscriptions.
If you have an Exchange-only plan, you can still get the shared Inbox and shared Calendar features of groups in Outlook, but you won't get the document library, Planner or any of the other capabilities.
