If you have a Scout who forgot their Scoutbook login, they should go to scoutbook.scouting.org and click “Forgot Password?” If he or she has access to the e-mail in the account, they will get a note with the ID.
If the parent (or leader) doesn’t know the e-mail used, use send mail, select both the parent and the Scout but DO NOT select BCC. It will send an e-mail with the Scout’s address exposed.