If they are truly just investors and not contributing as founders I would suggest structuring it as an investment. This does mean registering as a corporation. Registering your company is not all that difficult to do on your own.

If your concerned about the legal fees associated with properly setting up investors I would suggest checking out a SAFE (simple agreement for future equity). The SAFE is investment tool developed by Y Combinator, you can find more info at https://www.ycombinator.com/documents/#safe

As a disclaimer, it's always a good thing to speak with a Corporate Attorney before making any agreements or taking on investments.