Use Contact in Admin page
External Member and Group
You can add an external member to a group
Groups and shared Mailbox : same features
- Users can send-as or send-on-behalf of a Group mailbox, just as they would a shared mailbox.
Office 365 Groups : additional features
- Users (members) can subscribe to receive a copy in their own mailbox of the emails sent to the Group mailbox, which makes Groups closed to distribution lists.
- Groups include additional apps as a SharePoint team site, OneNote notebook, Planner, and Teams.
- Groups have a guest access model for external collaboration that shared mailboxes do not (Contacts).
shared mailboxes : additional features
- Shared mailboxes can have sub-folders in the mailbox, whereas Group mailboxes can’t.