Since iOS 9, testers are required to trust your organization’s Apple Enterprise developer certificate before running Beta distributions. This is a one-time process.
When running an app from an untrusted certificate, testers will see the message Untrusted Enterprise Developer. They can trust the certificate by following the steps below:
Go to Settings > General > Device Management on your iOS device.
Locate and select the developer profile under the ENTERPRISE APPS section.
Tap Trust "[Developer Name]".
Select Trust to confirm.

Remember to only trust Enterprise developers that you know and trust.