It's not clear to me why you don't simply invest in the business and be added to person A's LLC as a second member.
The key point for your question on this forum is that the IRS does not recognize the existence of LLCs, since they are state entities. A single member LLC is taxed the same as a sole proprietorship, and a multi-member LLC is taxed the same as an unincorporated partnership (unless, in either case, the LLC makes an election to be taxed as an S-corporation which is a federal entity). As a result, this tax forum is really the wrong place to even ask the question.
So there could be 50 different best ways to become a part owner or "buy in" to this business, and the best place to get advice is from attorneys and CPAs in your own state.
