Some apps cannot be downloaded directly from the App Store and need to be side loaded from a direct URL link. For those apps, please follow the below instructions:
1) Click on the "Download on the App Store" link at the bottom of the user invitation.
2) Users will be prompted to install the app. Tap the "install" button in the pop-up window.
3) When the app is launched, users will receive an "Untrusted Enterprise Developer Message". Tap the Cancel button.
4) Users will need to navigate to the device Settings app and open it.
5) Scroll to General - Profiles and Management - and accept the Enterprise app for the developer you wish to download the app from.
6) Tap the "Trust" message for the developer group you wish to download.
7) This will prompt another window. Tap the "trust" button in the pop-up window.
8) Navigate back to the app. Tap to launch it, and users can now log in.