Merge with a company with a set of ideas.

Hi! I met a few months old startup that is gaining momentum and have good technology and user base. I think that I can add a product to their offerings which can set them apart from the fierce competition. They obviously have all the ingredients to implement my product overnight. Since I would like to join'em as a co-funder and offer some tech help in the future, what should I do to avoid being ripped off? or is this completely silly.. Thanks