Before Xcode 6, it was sufficient to provide only a *.p12 certificate and *.mobileprovision file to Xcode in order to export an *.ipa file for Ad Hoc builds.
Xcode 6 ope
In fact, you need to create a new Distribution profile, specific for Ad Hoc Deployment. This can be found in the classic member center, but it is a new type of certificate.
You can then select which devices can be used to test the app as ou would do with a developer profile.
Alternatively you can use the TestFlight solution provided by Apple with iOS 8 to enable your user to have access to prereleases.
