You need to build your community from the get-go. Start with your beta testers, creating a community where they talk to each other about issues and come to you with ideas, suggestions and bugs. Encourage them to do this, no matter the issue, and respond quickly.

If you develop a community like this from the start, it's very easy to get your users to give advice on improvements and the like. You'll quickly see the beta users who are very involved, and ask them privately for suggestions of other beta users.

Beta users WANT to provide feedback, and they will provide some of the must useful feedback you can get, so long as they see you responding and taking into consideration at least some of their ideas. If you start this in beta, you'll build a community of users who want to help you succeed and want to help you improve your product, for the long haul.