As you may be already aware, there has been a website on which you can register to join Skype for Business preview (https://www.skypepreview.com/).
Well, now you also have the same possibility for Microsoft Teams. You can register at the following address http://aka.ms/JoinTAP100 to join Microsoft Teams preview programs.
By joining this preview program, you will be able to get ahead of everyone new Teams features, evaluate them and provide feedbacks to help shaping the product.
At the time of writing this post, it seems there was some issue with the registration form as it was stuck in ‘loading’… 