As per the documentation for the Get Users endpoint Reference | Twitch Developers if you make a request just to ‘https://api.twitch.tv/helix/users, it’ll return the user associated with the OAuth token used. If you want to retrieve details of other users you need to use the id or login params.
Also, you may have difficulty trying to build a tool to find similar streams based on a shared audience, as there is no possible way for you to find out who is viewing a stream. You could use follows but that can get into tricky legal issues if you are tracking other users and their relationships to other streams without their consent (or way to opt-out), and that’s also not the most valuable metric as many people follow streamers but rarely watch them, so follows are not exactly indicative of audience.