Adding Groups/Interests to SignUp at Checkout

I am just using this Plugin to allow customers to signup at Checkout – however the business has 3 distinct type of customers. I have created an Audience Group called Interests and a Checkbox for the 3 interests – so customers on a standard signup form can choose their interests before signing up. How can I add this checkbox to the Checkout signup process so customers can choose their interests when signing up at Checkout?

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