I believe if you try to register another account, you'll get an error message like, "Card already in system," even without a promo code.
The codes are for new customers only, so that wouldn't work anyway. Even if they're paying you, you would still be DH's customer.
You could resell the space under your account, if you want to be responsible for their content. Another option would just be to send them to sign up on their own with your promo code.
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