If your strategy is to market using bottom-up PR, you may need to bone up on your one-on-one marketing skills. The most straightforward way to recruit a small but enthusiastic user base to co-market your product is to convince individual users to try your product and hopefully evangelize it. Mass marketing campaigns won't do the trick. Even just getting the word out won't by itself do it. You need to "sell" the product and the co-marketing enthusiasm to each person.