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
Edit: A long time has passed since this answer. As for now I suggest using Fastlane for building the app from command line - with proper configuration it won't require signing with dev account.
Original answer below:
I don't know how to do it in xcode, however - I've managed to do it using command line tools:
xcodebuild -configuration Release -scheme SCHEME_NAME -workspace Workspace.xcworkspace clean archive -archivePath build/App
xcodebuild -configuration Release -exportArchive -exportFormat ipa -archivePath "build/App.xcarchive" -exportPath "build/App.ipa" -exportProvisioningProfile "PROVISIONING_PROFILE_NAME"