This is a “hidden” feature that can only be added via PowerShell at the moment. And web admin portal won’t reflect the changes you made via PowerShell. That means you can only view alias for Office 365 Group from the following PowerShell cmdlet.

First log into your admin account with PowerShell

Set-UnifiedGroup –Identity [email protected] –EmailAddresses @{Add="[email protected]"}

Check if it works

Get-UnifiedGroup –Identity [email protected] | FL EmailAddresses

EmailAddresses : {smtp:[email protected], SPO:SPO_fa5eb50c-147e-4715-b64b-76af8be79767@SPO_f0712c15-1102-49c4-945a-7eda01ce10ff, smtp:[email protected], SMTP:[email protected]}