Students can also log in with their username if they can't remember their email address for any reason 

Organisation owners are able to see the usernames in the Organisation  by Viewing Members either in the Students group if they are members of a course or the Members list if they are not
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 If though, you are not in the owners group but you are a teacher in the course, go to the course where you know this student is, and you can Access Student Code and you will be able to see his username in the browser