Back in December, Microsoft teased a smart speaker that would be powered by its digital assistant Cortana, and made by Harman Kardon. When Harman Kardon formally announced the Invoke, it said that the device wouldn't be available until the fall; however, if you're among a select few, you can get one now.
Microsoft and Harman Kardon are performing private beta tests in a program called the Cortana Friends & Family Trial. Unfortunately, we lowly non-testers won't be able to access the UserVoice forums, but the 'Knowledge Base' and 'How to be a great beta tester' sections of the site are available for all to see.
The ideas behind being a great beta tester are exactly what you'd expect from any such program from Microsoft. For the most part, the company wants the user to provide feedback, saying, "Giving us feedback within the Cortana application experience is the best way for us to improve the product. We want to hear your experience with Cortana, so don’t be shy be active [sic] and go to the Cortana App and submit feedback via the History feed."
There are also "surveys and homework assignments", which are meant to help testers to explore the product more. And Microsoft asks that testers "please be flexible, and patient as we continue to make development changes"; indeed, the product isn't complete yet.
The Knowledge Base page provides information about the product, such as device setup, general information, tips for reminders and calendar, and more.
Again, this is a private program and there's no obvious way to apply for it. There is a 'Contact Us' tab, which allows you to send a message, but that's it. The rest of us will just have to wait and see what we get in the fall. According to sources, the companies have already begun sending out units.