In order for users to install and use Private App store apps on their iOS devices, they will need to go through a process of trusting the publisher of the app.
Note: The Developer name that appears in iOS prompts will correspond with your Enterprise Apple Developer account name. The following screen shots feature 'Bizness Apps' as this is the Enterprise Apple Developer Name.
1. Grab the Private App store URL from your email and open the link in Safari on a iOS device. Click Install Now.
FOR iOS9.2 and UP, Tap Settings > General > Device Management. Under the "Enterprise App" heading, you see a profile for the developer. (Apple changed "profile" to "device management" in 9.2)
They will then click "Bizness Apps Inc." Then "Trust Bizness Apps Inc." Please note, "Bizness Apps Inc." will be replaced by your Enterprise Apple Developer name.
Feel free to save the image below to send to clients: