The Unity WebGL builds are intended to be played from websites. They don't work when loaded from a
file:/// url. When you launch a WebGL build from inside Unity, then Unity actually starts a tiny webserver to host the game, so your browser can load the game from
http://localhost/. So when your testers want to do that too, they need to set up a local webserver. Which might be too much of a hurdle for less technically inclined playtesters.
But there is an easier way: Just upload the WebGL build to a website and send your testers the link.
One great website for that purpose is itch.io. In order to publish a game you need to register with a valid email address. If you don't want to publish your build to the world yet, set your project "Visibility & access" to "Restricted" and either set a password or ask your testers to register accounts on itch.io so you can whitelist their usernames.
But if you also have an own website, then you can just upload the folder to it.