To distribute an iOS application to a private distribution group that includes unprovisioned devices via App Center, what step is required on the Apple side?

To distribute an iOS application to specific devices, especially unprovisioned ones, Apple's security model requires these devices to be explicitly registered with your Apple Developer account. This registration links the device's Unique Device Identifier (UDID) to your developer profile, allowing you to include it in a provisioning profile. Without this step, the application cannot be signed for installation on that particular device, regardless of the distribution method (like App Center). Adding the device owner to an organization or collaborators group in App Center manages access to the distribution platform but doesn't fulfill Apple's device registration requirement. Creating an unsigned build would result in an app that cannot be installed on any device, as all iOS apps require a valid signature.