This should be it

<?php 

function new_contact_methods( $contactmethods ) {
    $contactmethods[\'signupdate\'] = \'Date\';
    return $contactmethods;
}
add_filter( \'user_contactmethods\', \'new_contact_methods\', 10, 1 );


function new_modify_user_table( $column ) {
    $column[\'signupdate\'] = \'Date\';
    return $column;
}
add_filter( \'manage_users_columns\', \'new_modify_user_table\' );

function new_modify_user_table_row( $val, $column_name, $user_id ) {
  $user = get_user_by( \'id\', $user_id );
  $date_formatted = new DateTime($user->user_registered);


    switch ($column_name) {
        case \'signupdate\' :
            // return date_format($date_formatted, \'Y-m-d H:i:s\');
            return $date_formatted->format(\'Y-m-d\');

        break;
        default:
    }
    return $val;
}
add_filter( \'manage_users_custom_column\', \'new_modify_user_table_row\', 10, 3 );