PS HarriJaakkonen :~/Blog/Posts> cat ./office-365-group-and-alias-email-address.html

Office 365 Group and alias email-address.

If You want to archive this You have to use Powershell. Connect to Exchange Online Powershell and use the command below:
Set-UnifiedGroup -Identity GroupMailboxIdentity@domain.fi -EmailAddresses @{Add='alias@domain.fi'}
Happy Powershelling,