Although the build is available on the Xbox Insider Hub app, which is from the Microsoft Store, or on Steam, everyone can get either, regardless if they are an Age of Empires insider or not.
Those who want to join Age of Empires 4’s technical stress test do not need a special invite to access it. “ We are gearing up for a large scale test that will allow you to get hands on experience with Age of Empires 4 prior to its release date.” “ If you are reading this, then you are invited to join the Age of Empires 4 Technical Stress Test from Friday, September 17th at 10am PT, through Monday, September 20th at 10am PT,” the devs wrote. Of course, they are hoping that this will not be the case. Now, Microsoft has invited everyone to join in and rally as many troops as they can to figure out how much stress the game can take before its online infrastructure starts to crash down.
Age of Empires 4 devs just ran a closed beta test for a selected few last month.