External Users:An external user is someone outside of your organization who can access your SharePoint Online sites and documents but does not have a license for your SharePoint Online or Microsoft Office 365 subscription. External users are not employees, contractors, or onsite agents for you or your affiliates.
To view all the external users in a SharePoint online tenant, go to Office 365 Admin portal.
In the left side navigation, expand EXTERNAL SHARING and click External Users. Now, you can see the list of external domain users who are received invitations to access your sites and documents.