How to see a subusers username and password

You cannot see your sub users' usernames and passwords. You can only see their email addresses, because sub user sharing of forms is done by entering their email addresses:

https://www.jotform.com/help/232-How-to-Share-Forms-with-a-Sub-Account-User

Only the owner of the account, whether regular user or a subuser, ever has access to their own password.