When you host a Duo Deal, you first buy a deal with the discounted Duo Deal price, then you will need to invite a friend to buy that deal together with you. If you cannot find a friend to buy together, your hosted Duo Deal will be cancelled and refunded.
When you join an existing Duo Deal, it means you are accepting the invite from the Duo Deal host. You already have access to the Duo Deal price then, so you only need to complete the checkout. Once the checkout is completed, both you and the host will have your Duo Deal orders processed and prepared for shipping.
For more details and guidance, please refer to the official Duo Deal guide.