Look on Google Communities for technology focused BETA testing groups...

Look on Linkedin Groups for business focused potential testers...

Look on Facebook Pages for people who like apps/companies like what you are trying to bring to market...

That should get you started.

It sounds like you want to get some feedback beyond just UX testing? If so, then your best bet is to find some customers - even better, some paying customers. Offer an introductory rate and keep the numbers small. The introductory rate could be free but better if you charge for it as you'll get a stronger qualification of the people giving feedback. You can get the initial group by being active where they spend time online, or build a community around their shared interests within appropriate platforms for the audience you're trying to target. Consider using paid ads as a quick way of getting the initial interest but don't promote the app directly - promote the shared interest first - if they engage with you, then they'll naturally move on to try the app. Within Facebook ads for example, you can easily target competitors / similar communities. Another way might also be to approach an existing app owner with an established community where you can naturally offer value to their audience but don't conflict or challenge what they offer - the owners/ team might also fall into the testing group and they can also tell you how they built their initial testing group before they launched...I hope this helps - cheers, Ben

I like Dan's answer, and I'd also suggest you check out:
- Other social sites like Twitter and Meetups.com
- Online community testing portal Ramen- https://ramen.is/

Above inviting individual people, you can give people an incentive to sharing your app within their community. i.e. Invite 5 people to unlock a level, connect your twitter to get more credits, etc.