I had a friend approach me about this. I'm not a RE investor and know nothing about it. He is well established with many properties, but was short on capital for a deal that he wanted to pursue. He had proposed an agreement that would create a limited partnership in the rental with ownership based upon the percentage of capital contributed. Management fees would be paid to the company managing the rentals (which he presumed would be his company).
I ultimately said no because I don't know anything about the business and wasn't ready to plunk down $50k to move forward, and it didn't look like I'd have any involvement in daily operations (which is normal in limited partnerships I think). The approach he laid out did look fair to me, and did appear to protect both of us should the other flake. Still, the mortgage is joint and several on a partnership, so it looked like too much allocation to the pool for me. Because I knew I was out regardless of the arrangement, I never took it to a lawyer, but it would have been my next step, as well as talking through the agreement with another uninvolved friend in real estate to get her opinion of the arrangement.
Mostly: find someone that can review the arrangement and make suggestions, then get a lawyer to make sure it is all soundly crafted.