If you have uploaded your ipa file to appstore you can now test it using testflight.
2. Click on My Apps and select the app you want to test.
3.Click on Testflight and choose add itunes connect users
Add your team members here to test the app.You can add up to 25 internal testers. They will get an email once you add them.
5. Install Testflight application in your iphone. You need Testflight application to test the app in your device.
6.Testing the app in testflight is simple for your developers.When they receive the invitation they just need to open the invite in testflight.
7. If you need to send invitation for external users for beta testing check out this video.