TestFlight in iTunes Connect now provides multiple build support, enhanced group capabilities, and improved tester management—making it even easier to test your apps.
TestFlight now lets you distribute and test multiple builds at the same time, so testers can choose from a number of builds to test.
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.
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 and you can easily resend invitations to testers who have not yet accepted their invitation.
Source: Apple Developer