It seems that Google may start doing beta tests for games on Stadia. Not necessarily directly though. Instead the beta sessions would go through GBTN.
If you're unfamiliar with GBTN and you have a Stadia account, you'll want to take note. GBTN is a service called the Global Beta Test Network, and its purpose is to get people to signup for beta tests of games on various platforms.
Once they have people signup for the beta tests, they send logins and any other related info to the users who are accepted to participate. The users then test games, and then report feedback. Much like any other beta test.
Stadia beta tests may not happen at all
GBTN has recently sent out emails to its users to let them know that sessions requiring a Stadia account may happen.
This suggests that beta testing of games may not happen at all. At least not for a little while. It is possible though to prepare for the potential availability. First what you'll need to do is setup a GBTN account if you don't already have one.
If you're already a member of GBTN and have participated in previous test sessions, then you're halfway there. Once you have an account setup, or once you login, you can now add your Stadia account, and Stadia controller to your GBTN profile.
This will allow you to opt into any beta test sessions for Stadia when and if they arise at some point in the future.
Adding your Stadia account to your GBTN profile
If you already have a profile set up, then adding your Stadia account is easy. On the main GBTN page after you login, click on the Testing Info tab up top, then click the dropdown option and select Stadia which will be at the very bottom of the list.
Select it, and then click on the space next to it and enter the email that's associated with your Stadia account. After entering that in, click the add button. That's all there is to it for this particular section of the process.
You will also need to add your Stadia controller to the profile, as this is considered the "platform" which you'll play games on. Click on the platforms tab all the way to the right. Once it loads, click on the big "Add a Platform" button then again select the dropdown, and select Stadia Controller.
Once you're done with that step, you're finished. All there is left to do is wait and see if you're contacted regarding beta test sessions that require Stadia. Keep your eyes on your email associated with the GBTN profile, as this is where you'll get notifications about possible tests.