Many new software companies invite individuals to sign up for their beta program. This means the user will experience the app before other users and will have private access to the APK (Google's Android Package) that contains the app itself. However, APKs sent via email can sometimes be fickle when installing. This is why adding APKatcher to your phone will help eliminate this issue and give you two options for installing apps from email.
Step 1: Install APKatcher from the Android Market.
Step 2: Open the e-mail with attached APK file.
Step 3: Click on the Install button at the bottom of the e-mail.
Step 4: Choose APKatcher from the pop-up menu.
Step 5: Read over the app's permission requests and choose whether to install it on your phone.