How do I create a customer portal for a specific customer

We are a gifting company and want to be able to offer a link to a customer that they can email a gift recipient to redeem a gift on our site. How do I do this in shopify? Since our customer (not the recipient) will be paying for it, we want to make sure that the gift is designated to a specific person and that they can only claim what we specify (qty, size…)

Any suggestions??