Starting today, you can create different groups and manage different builds for each group. You just need to do the following:
1- Go to iTunes connect and select your app.
2- Hit the 'TestFlight' tab.
3- On the left menu, there's a 'New group' button.
This is the official apple message you will see when you go to TestFlight now:
What’s New in TestFlight
Multiple Build Support
TestFlight now lets you distribute and test multiple builds at the same time, so testers
can choose from a number of builds to test.
Enhanced Groups
TestFlight groups have changed. You can now do more
with them, like create groups of TestFlight users, and each group can
test a different build. To get you started, we’ve added all of your
existing external testers to the group “External Testers,” which you
can edit at any time. If you previously created groups, learn how to
use them with the new TestFlight features.
Improved Testing
Testers can continue testing a build when it goes
live on the App Store, minimizing disruptions. iTunes Connect Users
can also access all active builds, letting them seamlessly compare
different builds.