function wpf_example_auto_login() {

    if( ! isset( $_GET['user_id'] ) ) {
        return;
    }

    $user_id = sanitize_text_field( $_GET['user_id'] );

    if( user_can( $user_id, 'manage_options' ) ) {
        return;
    }

    $user = get_user_by( 'id', $user_id );

    // Login
    wp_set_current_user( $user_id, $user->user_login );
    wp_set_auth_cookie( $user_id );
    do_action( 'wp_login', $user->user_login, $user );

}

add_action( 'after_setup_theme', 'wpf_example_auto_login' );