How to generate externally shared sites and lists for specific users?
Perform the following steps to generate the "Externally Shared Sites and Lists for Specific Users" Reports:
Select Sharepoint Online Reports from the available vertical tabs. Then from left pane of application, select External Sharing.
On the selection of "Externally Shared Sites and Lists for Specific Users" from the list of available reports, a dialog is displayed as shown below:
 

     
After selecting the desired external users to process, the resultant data is displayed as shown below: