When my app goes on sale, I'd like to let a few selected customers who helped with development, have it for nothing. Is there any way I can do this?
Can I give away my not-free-app to specific users who helped with development
Yes. You can generate up to 100 promo codes per version of your app. https://developer.apple.com/library/ios/documentation/LanguagesUtilities/Conceptual/iTunesConnect_Guide/Chapters/ProvidingPromoCodes.html
You can generate and distribute promo codes, but remember, they are only good for 30 days, that version, and recipients can't leave reviews.
If there are only a few of them, you can simply gift them your app. Then they have all the rights and privileges of regular app buyers (can leave reviews, never expires, etc.).
How do you "gift" an app?
On the App Store app on a device, go to the app's page and tap on the Share button (looks like a box with an arrow pointing up) in the upper righthand corner. Gifting the app is one of the choices on the Share menu.