I'm interested in testing apps for developers but I don't know how to find the sign up for that? I need help, by using the test flight app
How do I Test apps for developers? Using test flight
To use TestFlight you have to join the Apple developer team ($98.99 annually). To do this download the developer app and go to account and it should give an option to sign up there.
Note that decision to invite someone to participate in TestFlight for an app is a developer's decision. So, you would have to be invited, either directly or through a public link provided by developer. https://developer.apple.com/testflight/