How to find a site users contact info

I have a Site but I want to be able to contact people who visit the Site.

Obviously this is all taking for granted that they've agreed to the terms and conditions and we're not violating GDPR and they've given their permission to recieve marketing materials etc,.

At present, I'm just getting the users IP like so:

if (!empty($_SERVER['HTTP_CLIENT_IP'])) {
    $ip = $_SERVER['HTTP_CLIENT_IP'];
} elseif (!empty($_SERVER['HTTP_X_FORWARDED_FOR'])) {
    $ip = $_SERVER['HTTP_X_FORWARDED_FOR'];
} else {
    $ip = $_SERVER['REMOTE_ADDR'];
}

And am using this for geolocation to serve the Site in their country.

What (if any) tools would be able to give us contact information for specific IP addresses or what other information would I need to get a users contact info, such as an email address.