You can resend the Welcome Letter by going to the Welcome Letter and Invoice Settings tab on the family page. 
  1. Go to the family's account
  2. Open the left side menu by hovering over the white arrow
  3. Click on Welcome Email & Invoice Settings
  4. Under Welcome Letter and Invoice, click Resend Welcome Email.
Any adjustments that were made since the last Welcome Email was sent will be reflected on the revised Welcome Email.

You can view the revised Welcome Email by going to the family's Communication History, found under the left side menu.