A service like HockeyApp is great for distributing to a limited group, but if you want to do real enterprise distribution you have to be the curator of your own "store".
It's actually very simple.
You need to have some page that hosts a link to your distribution. The link is of the form:
itms-services://?action=download-manifest&url=https://<path to manifest file>/<manifest file name>.plist
The manifest file is a property list file that gives necessary details about the app.
It is of the format:
<plist version="1.0">
<dict>
<key>items</key>
<array>
<dict>
<key>assets</key>
<array>
<dict>
<key>kind</key>
<string>software-package</string>
<key>url</key>
<string>
<path to ipa file>.ipa
</string>
</dict>
<dict>
<key>kind</key>
<string>full-size-image</string>
<key>needs-shine</key>
<false/>
<key>url</key>
<string>
https:<path to icon file>/iTunes512.png
</string>
</dict>
<dict>
<key>kind</key>
<string>display-image</string>
<key>needs-shine</key>
<false/>
<key>url</key>
<string>
https://<path to icon file>/Icon57.png
</string>
</dict>
</array>
<key>metadata</key>
<dict>
<key>bundle-identifier</key>
<string>com.your.app.id</string>
<key>bundle-version</key>
<string>##</string>
<key>kind</key>
<string>software</string>
<key>subtitle</key>
<string>
Subtitle for app.
</string>
<key>title</key>
<string>Title for app</string>
</dict>
</dict>
</array>
</dict>
</plist>
It points to the location where your app is hosted.
The ipa file and all of the icon files have to be behind https.