So, you have a field with new user's email account. You'll also want a status field which you set to Submitted when you want to kick off the email.
Promote both of these fields - File -> Form Options -> Promoted Properties.
Republish your form to SharePoint.
Now, create a reusable workflow that emails a notification when the form's status is equal "Submitted" (condition). You can get the new user's email from the Current Item's properties because like the status it's promoted.
Piece of cake before breakfast...