Typically this type of stuff would be covered in an operating agreement. It sounds like you're trying to run pretty lean right now, so hiring a lawyer to help with an operating agreement could get expensive. I suppose you could do it cheaply by starting from a template or using something like LegalZoom, but typically you pay for what you get.
You'll also probably want to be careful not to run afoul of any labor laws. In most jurisdictions in the US, anyone that can't be classified as an independent contractor must be paid at least the minimum wage. I'm not sure if listing someone as a shareholder in an operating would cause problems with that. I believe the only exception to paying minimum wage in many jurisdictions is if the employee has a stake of X% in the company (where X is something like 20%).
Of course, if you really trust the people you're working with/hiring, you could get by on a handshake deal for a little while until some revenue or investment money comes in. I've done this a couple of times in the past myself, but there's always more risk in doing that way.