If you’re just now signing up for the Android Nougat beta, you’ve run out of luck. That said, you can still sign up for the Android Beta Program in due time to track the progress of Android O development.
The first Android O image was delivered late in March and flash-only for developers to get just a taste of what Google wants to bring onto the table for Android. We expect that an update to the beta image will come in a week or two, perhaps right at Google I/O 2017 on May 17. It might be around this time that Android Beta Program participants will get their first OTA update for Android O.
Check out our Android O overview — even though it’s quite the iterative update on its face, there are some nifty features you should be familiar with.