There are 3 WAYS to create .ipa WITHOUT Command & Apple Developer Account.
1. Fast & Best
(Works on all Xcode, All Mac OS, Bundled package can be used for OTA links like Diawi)
- Just build (Command+B)your app from XCode by setting proper code signing identities
- From XCode'sfile search at left bottom, search for.app(This will be under product directory)
- Right Click on this .appfile and select Show in Finder
- Now, create directory and name it as Payload, copy .appinto Payload directory.
- Archive/Compress(.zip) this Payload directory, rename file extension from .zip to .ipa
2. Extract .ipa from organizer
(Works on all Xcode, All Mac OS)
- Create Archive
- Go to Organizer
- Get location of Archive by Right Click and selecting Show in Finder

- Now right click on this .xcarchive file and select Show Package Contents
- Go to path - Products > Applications > YourAppFile
 
- Open iTunes's on Mac and drag-drop this  YourAppFile from above path in Apps tab. (NOTE: Delete the previous app with the same identifier if any) 
- Now right click on your app under Apps tab of iTunes and select Show in Finder, this is your .ipa file !!! 
3. For devs having Xcode 5.x
(NOTE: Xcode 5.x doesn’t work on El Capitan, By this way you can archive using any Xcode but you will need Xcode 5.x to create .ipa)
- Create Archive using any Xcode version.
- Close Xcode.
- Go to Organizer window of Xcode 5.x and from Archive tab select your archive.
- On top right section select Export, click radio button with titleSave for Ad Hoc Deployment
- Click Nextand Select desired code signing identities to save .ipa