My team depends on testflightapp.com for ios testing. Now this site will be closed by next month and we should use itunes connect.
We had made some research and found that itunes connect will send release details via email. And testers needs testflight app in their device to download our build.
Now the problem is, testflight app is only compatible with ios8. Does that means our testers cannot download and test our app in ios 7 devices?
That is correct. You could use a similar service like Crashlytics or Hockey App, although there may be a charge for the service. You can use the "over the air" functionality on your own site if you wish. Mac Apps like BetaBuilder will help you generate the required files.