To put an app on your phone you have a few options.
1. Put it in the store (you have already said that you don't want that)
2. Put your app into Cydia
] (but this only works for jailbroken phones)
3. Make a mobile website app that people can just run from their browser and then add to their app screen. (This is not taking an existing one but redonig it as html5) A sample of this is here Kendo UI
4. Have an enterprise license for the company that makes that app and then they can install it on machines outside the store. This might involve actually getting the device id for each device it is installed on.
5. Have a provisioning profile for the app with all of the possible 100 device ids that you can enter into your account and then they can download the provisioning profile and the app and install them both with iTunes.
These are really your options for an iPhone app. I would recommend getting the html5 for the app (if it was written with that) and post those as a website that people can anchor to their screen.