Hello, I wonder how to share novels  to beta testers or other people who doesn't have accounts on the site. I know the game has to be unlisted but I shared the preview link of the game with a friend and she when clicked it, it told her to sign up. Am I doing this wrong or is this not possible? I also tried the link in the settings but she was still told to sign up or login.

You will have to publish your visual novel as unlisted, then share the published link (just copy and paste from your browser address bar) to your friend. If you share the preview link (aka when you are in edit mode) they will have to have an account and added as collaborator under settings.

So I would have to publish my novel to get the link, correct?

yes, you publish it as unlisted. unlisted sort of means private. only people who have the link will be able to see it. 
when you publish as unlisted, you just treat is as a normally published game, so you go to your dashboard, click on play, get the link from browser