We’ve had success with TestFlight and BuddyBuild for providing access to beta / pre-production apps for our Testers to install.
In the case of TestFlight, you will need to provide an invite link in the environment to allow our testers to install the beta app you would like to test. You can find more information on the process of creating an invite link on this page from Apple, under the heading “using public links to invite testers”. Unfortunately, we do not currently support inviting testers by Email, in order to preserve the privacy of our Testers.
Similarly, if testing an app via BuddyBuild, you will need to provide the BuddyBuild link to install your beta app under the environment. As BuddyBuild provides a choice of multiple versions to install when you provide the link to your app, it is important that you also indicate exactly what version of the app you would like the tester to install.