Merge with a company with a set of ideas

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

The short answer is there is nothing you can do to avoid being 'ripped off' (barring all sorts of expensive and complicated legal paperwork which they will almost certainly say no to).

Let's ask two different questions. You want to be a co-founder with them:

1. Do you bring something to the table, besides the idea? Would it be smart of them to want you as a co-founder? If you bring value as well as the idea, they would be stupid not to take you.
2. Do you trust them and want them to be partners (if not, walk away)? If you trust them and are wrong, do you really want to be a co-founder with people who will steal your idea? If they steal it, maybe you should thank yourself for avoiding years of pain with them, and chalk it up to a mistake in reading people.

The best thing to do is to have what negotiators refer to as a BATNA: have an alternative course of action planned out if they do steal it ... or start the company yourself.