You can make a Dashingdon link private and just share it with the tester group if you like, though it’s not fully secure if someone shares it outside the group. Moody has more robust private settings but seems less stable unfortunately; you can also compile a ChoiceScript game and put it on itch.io as a private or password-protected game.
My experience is CoG-specific where the way it’s usually done is to have a private beta once the full draft is complete. When putting up my WIPs in public I’ve generally had more detailed feedback at the beginning, and then in the private beta test there is an uptick in that sort of feedback again as people have the bigger picture/perspective having played the whole thing.
I know authors have had good experiences with both fully public and partially-private playtesting. There was some very interesting discussion in this threadv which gave me a lot of things to think about: