Until now, the Play Store beta testing system was tied to Google+ communities or Google Groups, but that's changing now. Google is rolling out two new options for developers to run beta tests that don't rely on Google+.
The open beta system will still use testing links you're familiar with from the Google+, but you won't have to join a community. Anyone with that link can join the test, but developers can place a cap on the number of testers. The second new system is closed betas using email addresses. Developers will be able to add users individually or by uploading a CSV file. This is fine for small groups of testers as you don't have to set up a community, but it won't scale well to larger tests.
The standard closed beta tests with Google+/Groups will continue to work as usual, and I suspect many developers will still use that. It provides a convenient place for users to report bugs and ask questions.
- Android Developers