Google has a new preview program in place, and you can join it right now if you're interested. The 'Chromecast Preview Program' as it's labeled by Google allows users to test out new updates for the Chromecast before the software is pushed out to the public, and is meant like most programs of this nature to allow users to test new features before getting a wider rollout to the general populous of Chromecast owners. Applying to join the program is fairly easy and can be done in just a handful of steps, beginning first with turning on any device you have a Chromecast plugged into, then going into the Google Cast app and tapping on the devices tab, then device settings, and finally on "preview program," which should be listed unless Google is no longer accepting new members.
If you see this option in your device settings, then you can sign up to be a member of the preview program and Google will end up sending you a confirmation email, after which you'll receive emails when software updates are ready for testing. These could be updates aimed at helping Google test enhancements to the device through improvements, or they could be updates aimed at helping Google test new Chromecast features. Whatever they are though, Google will detail this information in emails that are sent out to members. Having said that, Google also makes it possible to disable email notifications with a slider bar that shows on this screen, however, disabling these notifications will also remove any possibility that you'll be alerted to when new updates are sent to your Chromecast device.
Google notes that these updates are not "beta" software and are simply early access updates that are designed with the intention of simply allowing users to test things early, but, also that the software will be of the same quality as any regular update that is pushed out to all users. As there are now multiple Chromecast products out there in the wild, users might be wondering if their devices are eligible to be a part of this program, and the short answer is yes, albeit this does not apply to Google Cast enabled products, and will only be for the 1st generation Chromecast, the 2nd generation Chromecast, and the Chromecast Audio. If you're interested in being a part of this program, open up your Google Cast app and see if you're eligible to join.