iOS app Distribution outside of App Store
Your question seems quite unclear as you seem to have given the answer together with the question.

The way to do this is to apply to the Apple Developer Enterprise Program. Using that account you can create an In-House Distribution Certificate that you can use to sign your app with in Xcode.

After you have signed your app, you'll need to create a manifest plist file. Host the app (.ipa) and the manifest (.plist) in a folder on a web server, and send the user's an invitation via email.

The user will then be able to download the app, trust the enterprise profile and eventually run the app.