Adding external emails to an Office 365 Group email

1. Log on portal.office.com with an admin account
2. Under Users -> Contacts -> Add a contact

3. After that you will able to add those email in the contact to the O365 group email (distribution list)

Reference:

https://answers.microsoft.com/en-us/msoffice/forum/msoffice_o365admin-mso_dep365/adding-external-emails-to-an-office-365-group/93acaab6-0b24-4bbb-b2c8-ac1a3f8c45a0

Adding external emails to an Office 365 Group email