Please use this below script to extract SharePoint Online Site External Users to an CSV file.
Get-SPOExternalUser -SiteUrl "Your Site Name" -Position 0 -PageSize 50 |Select-Object DisplayName,Email,WhenCreated,AcceptedAs,InvitedBy |Export-Csv -Path f:\script\externusers.csv -Encoding ascii
Get-SPOExternalUser -SiteUrl "Your Site Name" -Position 0 -PageSize 50 |Select-Object DisplayName,Email,WhenCreated,AcceptedAs,InvitedBy |Export-Csv -Path f:\script\externusers.csv -Encoding ascii
