What you need to do is email:
(or use the contact form at https://luma-touch.com/support
You will need to include details of what you can bring to the party ... being a Beta Tester is not just about getting a free/latest copy of the app
Bear in mind that a tester potentially has an app that is not fully working. Often when something new is added it breaks something else. Also features are tested but removed if they prove unstable or unusable. You could end up losing projects
Ha ha ha ... Grand Master ... thanks Rob!