Hi! Nowadays beta testing is completely open and public.
For PC, Mac and Linux, simply look out for threads in this community, announcing new beta builds. You can download them through itch.io
For Android, follow this link, and you will enable the beta builds:
The reason I have beta testing is to get feedback on builds before I release them. I have no professional quality assurance program for this game, and I rely mostly on you guys to help me. So please send me feedback when you are testing beta builds. Even a quick note like:
"I'm on Mac and this thing seems to work"
Is really useful!