When we were looking to fill up our private beta for AppThwack early last year we went to reddit, XDA, LinkedIn groups, and other places where we thought our target customers (Test engineers and app developers) would be. We posted non-spammy posts asking for beta testers and did not mention our product's name. We just described the problem we solved, how we were doing it, and asked for volunteers to sign up.

If I was doing it again I'd follow a similar path, but I'd also approach some of the more influential people in our target market and ask them directly for their help and feedback. We've done that with various features since and it's worked out great.