Anyone is able to beta test at any time they want, simply by cloning the repository locally. Although it should be noted that TSLPatcher is no longer supported and HoloPatcher is now the required installer. If you want to try out the current build and test for bugs then you need to be quick. I will be starting my second 1.10.0 beta test playthrough this weekend, looking to have the final release ready by Xmas.
Edit: As of this evening, I have wrapped up the last of the planned updates before I begin beta 2 this weekend. So if anyone wants to do a playtest, now is the time to jump in. But you'll need to be quick with your feedback, since I want to get the final release out by next weekend.
Current changelog for v1.10.0: