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
If you are using Testflight to upload adhoc builds, it's easier. Install the Testflight app for Mac and run it. Whenever you do an archive in Xcode, it auto shows a popup whether to upload it. You need not export the build from Xcode and avoid signing in to developer account.
Alternatively, if you don't want to distribute on Testflight, you can right click (or option click) on the build in Xcode > Organizer and select "Show in Finder" and right click the archived file for "Show package contents" and get the .app. Put the .app inside a directory called Payload and zip up the the file to Payload.zip. Rename Payload.zip to myapp.ipa to distribute manually.