You must download the assembly first. After you have done this, you must notify interested people (including yourself) by selecting either your UDID or email identifier) in your testflight account team. After both are done, you will begin to see assemblies.
UPDATE:
Once you sign up, inside your panel you should see the "Create Assembly" button. Click on it and it will offer to delete your .ipa file. Select and lower and click the Download button to download.
After downloading, the "Update Profile" button will appear. You must click it if you are downloading this application for the first time. Then upload your provisioning profile to testflight. I assume that you know the process of getting the profile file from Xcode.
This last one is a very IMPORTANT STEP, since all devices added to this profile will be automatically notified every time you download and update the assembly version on testflight. If they are registered users of testflight, you will see their names / emails. If it is not, you will see their UDID. But where will you see? When you download the assembly, you will see it on the Permissions page.
As soon as you click on each of the people and click on “Update” (just update) or “Update and notify” (update and send email if registered), you will begin to view assemblies on the testflightapp website.

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