Sharepoint online enables you to share (sub)sites, lists, libraries or files with external user, when the tenant administrator allows this feature.
Using Powershell, you can check the users that have access to a (sub)site. To do so, you’ll need the Sharepoint Online powershell module.
Once, connected, use the
connect-sposervice cmdlet to connect to the administrative site of your tenant (https://tenant-admin.sharepoint.com). After that, you can use the
get-spoExternalUser cmdlet to retrieve the external users known to the site.
As you can see, you’ll see who invited the external user and the live-id / account used to accept te invitation.