There are many options. You need someone to develop a partnership agreement and also work on a Visa for your Partner. Regarding the Visa, you could sponsor through an H-1B Visa or perhaps another depending on the facts of your case. My firm regularly works on Visas for start ups and also works on partnership agreements. Here are a few of our articles on the topic.
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There are a variety of ways to get this done in a way that is fair to both parties. Perhaps an incentive-based agreement. Perhaps a full partnership. Since you will be looking to sponsor them in the US, you have a certain amount of leverage when it comes to other terms in the agreement.