beta testers question

Well, how do you plan on finding users for your app when it is officially available? Apply those same tactics to getting beta testers.

hopefully you have created an app that people actually want, and already have an ideal user in mind. So go to where these users are. Wether it is a real life location, or an online community.

eg. If it's a photography app, join photography sites and forums and post there. Go to photography meetups, and groups and ask people in person. Go to a park, I'm sure you'll see someone taking a picture, just approach them and ask if they want to try your app.