How does private lending actually work? (Legally, taxes, etc)

[Ramsey Persing]

Hi, 

I'm new BP and getting things together for my first BRRRR. One thing I'm a little confused on is how Private Lending works. I have a few people that have indicated an interest to help me with financing in this, but I know there are limits on cash gifts you can receive so someone can't just give me $50,000 to buy a house and I return it to them in a few months with interest. So how is this usually done?

Do both parties co-sign on the deal then refinance to only one name in a few months? Does just the investor sign for the house then I buy it back it back with a conventional loan later at 75% of the value? 

I'd love to know how this works and what others do before I sit down and have the second conversation with the potential investors to make a clear plan. 

- Ramsey