I think you should turn to a professional (Lawyer) to figure this out. Securities, taxes, and company structure should be done properly - it can get you in trouble, and at minimal, make it harder to change or cause issues with what people thought they signed up for, but is now changed.
I understand your startup is early stage, but if you start to have full time employees or you have or planning to raise capital, then you should get some professional advice.
Having said that, this is a good blog post on these matters: