With the release of iOS 9, testers will be required to "trust" your organization's Apple Enterprise developer certificate, one-time, before running Beta distributions. Testers will see the following message when running an app from an untrusted certificate and can trust it by following the steps outlined below on their device.
Note: In the screenshots below, "Twitter" is the app being installed and the name of the developer certificate. The app and certificate name will be specific to your distribution.
1. Navigate to Setting -> General --> Profiles
2. Select the developer certificate in the "ENTERPRISE APP" section
3. Press "Trust [Developer]"
4. Select "Trust"