install Windows Phone 8 app to more than three devices without store

Question

I am developing an app which should be used in an company, internally only (about 100 installations).

I found out that I can install the app to the phone directly (without store) only if I register the phone as development phone. Since I can only register three devices, how do I install the app to the other phones?

The app should not be public, so I can not publish it to the store.