Find small blogs (10k-50k subscribers) relevant to your market and offer them 100-1000 signups with a custom-branded demo code. The blogger likes getting an exclusive for her readers, and the readers like getting insider access to a hot new tool. Contact a bunch of them at once, we felt lucky to have a roughly 20% hit rate.
Pretty creative, nice idea of bringing people on for beta testing your product. You know, once you release something, it's always a headache: "So what's up now?" Following this advice could give you at least a starting point.