Are you trying to show the the username on the form or are you trying to get the username in the form response sheet? If you just want the username in the form response sheet after submitting the form, then you can save the email special value to the sheet, then create a relation column to link the email in the form response sheet to the email in the user sheet. Then create a lookup to get the username from the relation.
I think that might be really hard to do at the moment. You would have to have a duplicate page for every user where the row has their email address. Then you could create a relation and lookup to the user sheet to get their name. But again, you would have to duplicate everything, so each user would have to have their own Pencil row and their own Apple row, etc.