Pitch deck advisor, startup consultant, profit model specialist. Founder of http://www.ventureformations.com . By the time I was 17 I had started and sold my first tech start-up. Through my experience and knowledge I am able to provide professional feedback on your pitch techniques and content to help you become more confident and comfortable when delivering the pitch to investors. "Powerful Confidence. A Persuasive Presentation. A Winning Pitch.”
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.