If you have a product many people are interested in, it's inevitable that a few will be willing to beta-test it, with all the implication (i.e. free, no-royalties). So, your beta testers are first and foremost your customers. I think it's obvious but worth mentioning anyway. People who don't care about your product won't care enough to beta-test it (unless it's paid QA staff).

One of the cornerstones of XP is frequent releases for quick interaction with users. User interaction is a great debugging tools - users will find bugs where all your testing efforts fail.

So, release it to the users. Let them test it and play with the system.