Collect beta signups via landing page with MailChimp form
Send out invite urls that have an invite code attached. Even though invite code looks random, everyone gets the same invite code. It just sets a cookie that's read client-side and renders the app if it's correct. No need to overengineer.
Invite beta participants to a private forum or Slack to give feedback.
Try to schedule calls with as many beta testers as possible to learn more about them and their experience.
Spend as little time as possible in beta. Launch publically a few weeks later unless there are major issues that need to be addressed.
My process:
Great! Thank you so much for your advice.
I am developing JPEG artifacts removing tool and for Beta version:
And yeah, for testing purpose I also publish demo-version with quite restricted functionality hoping someone will write a feedback :)