In your function.php add folowing code with all users email ids in args array as one mail id in one line. Or you can split the same using commas.

function add_user(){
$args = “sample1@sample.com
sample2@sample.com
sample3@sample.com
“;
$pieces=explode(“\n”, $args);
foreach($pieces as $piece) {
$username = $piece; //username
$password = $piece; //password
$email = $piece; //email
if ( !username_exists( $username )  && !email_exists( $email ) ) {
$user_id = wp_create_user( $username, $password, $email );
$user = new WP_User( $user_id );
$user->set_role( ‘subscriber’, ‘ ); //either of ‘administrator’, ‘subscriber’, ‘editor’, ‘author’, ‘contributor’
}
}
}
add_action(‘init’,’add_user’);

Reload the website.

Please not that if you have more than 500 users, first add 500 or less users at a time. Then replace the code with next part so on…