The Apple program that you should join if you're in charge of testing your organization's infrastructure and its ability to work with the Apple software before public release is Apple Seed for IT. This beta-testing program, available to any business organization or educational institution, allows you to evaluate the latest prerelease software versions in your unique work environments. You can test the compatibility, stability, and security of your IT infrastructure and business-critical apps with the upcoming releases of macOS, iOS, iPadOS, tvOS, and watchOS. You can also provide feedback directly to Apple and influence the development of the software1.
The other options are not suitable for this purpose, as they have different goals and requirements. Design for IT is a program that helps you design and implement solutions for Apple devices in your organization, but it does not provide access to prerelease software2. Apple Developer Program is a program that lets you distribute apps on the App Store and access developer resources, but it does not offer a dedicated channel for testing your organization's infrastructure3. Apple Developer Enterprise Program is a program that allows you to develop and deploy proprietary, internal-use apps to your employees, but it does not allow you to test the compatibility of those apps with the latest Apple software4.
References: 1: Apple Seed for IT - Apple Support. 2: Design for IT - Apple Developer. 3: Apple Developer Program - Apple Developer. 4: Apple Developer Enterprise Program - Apple Developer.