Do you have specific concerns about running a closed beta, or are you just generally trying to be prepared for it?

Ideally, you would know or seek out an initial group of youth ministers who might be interested in using your application, interact with them directly, perhaps conducting some usability testing in person, but also monitoring their use of the app and continuing to work with them directly throughout the beta.

If you are successful at convincing these ministers of the value of your web application, they will help you convince others, which is usually the best way to grow. You might even select some of these initial customers to join a customer advisory board over the long term if that ends up feeling useful.

If you have concerns about scale, you can always move from a closed beta to an invite-only beta that helps you manage scale, but I'm not certain that's going to be one of your immediate problems.