You will recieve an email inviting you as a beta tester via the testflight app. When you accept the invite you will be asked to connect the device. When the device is connected then you are ready to install the app via an install invitation.
You can give feedback at the combinedblog website. There is a custom feedback form there.
Alternatively you can reply to the testflight email which contained the install link.
If you want to follow the build process then check out the blog which will have updates on the app up to the full release.
Lastly give us some feedback on the proposed icon to start with.