If you use the ad hoc provisioning profile distribution, as you did, you meet at least two problems:
- A limited time your users can use the app (the ad hoc provisioning profile has a limited duration)
- A limited number of users can run the distributed app (remember that you can use at maximum 100 devices per year for testing, and from the Apple's point of view your users are just testers)
In general, to achieve what you want with iOS, you have a couple of options, but probably no one of them perfectly fit your needs:
- Registers with the iOS Developer Enterprise Program. In this case you can distribute your app without submitting it to the App Store, but this method works only if your users are employees of your company
Enrolls in the Volume Purchase Program for Business (VPP Business Guide). In this case you can submit your app, in a restricted number of Countries (United States, Canada, United Kingdom, France, Italy, Germany, Spain, Australia, New Zealand, and Japan), to a limited number of users, through a sort of parallel App Store.
This procedure is intended for companies that fit the following requirement: big companies (that have a DUNS number) that hire a company that develops a Business app for them (aka marks the app as Business and related to that particular company during the app creation in the iTunes App Store). The app will not be visible in the traditional App Store.
Maybe this second case can be your case, in case you are realizing the app for a DUNS company. Please consider that in this case the company itself (and not you) establishes who can install the app by sending it to the selected target users in different ways (promo codes, links to the app store,...).
I don't see other options at the moment.