If you release only a file.ipa to install that you need the jailbreak on the device, that's if is a big company is impossible, but you have a 2 possible ways:
1) huge an slow but is effective, finish you app and pass the project on a laptop, create new buy a new developer program only for this company, setUp the laptop with new certificate and install the app on all company device manually one by one, ins very slow and huge process, but it work, no app on appStore, and no body know that your project exist.
2) publish on app store with AdHoc provisioning profile, have 100 device per App, but you can publish more same app with different name ex: App1, App2 ext.
The app is on appstore, but not visible, only the device with AdHoc Provisioning Profile can install the app.
3) make a jailbreak on a device company heheheh
Hope this help you