Magento 2.1 Grouped Products – How to Get Custom Product Attribute by Product-ID in Grouped Product

1. please check the custom product attribute assigned corresponding attribute set.

2. To Display, the new attribute in List Page, set the value “YES” to “Used in Product Listing” under storefront Properties while creating the attribute.

3. To display the attribute value in product list page, add the below code in app/design/frontend/YOUR_VENDOR/YOUR_THEME/Magento_Catalog/templates/product/list.phtml.

for detailed answer please follow the below link

http://www.dckap.com/blog/display-custom-attribute-value-product-viewlist-page-magento-2/

Create di.xml file here in your custom module

app/code/Vendor/Module/etc/di.xml

Content for this file is..

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <preference for="Magento\GroupedProduct\Model\Product\Type\Grouped" type="Vendor\Module\Model\Product\Type\Grouped" />
</config>

Now you need to create Model file here in your custom module..

app/code/Vendor/Module/Model/Product/Type/Grouped.php

Content for this file is..

<?php

namespace Vendor\Module\Model\Product\Type;

use Magento\Catalog\Api\ProductRepositoryInterface;

class Grouped extends \Magento\GroupedProduct\Model\Product\Type\Grouped
{
    public function getAssociatedProductCollection($product)
    {
        parent::getAssociatedProductCollection($product);
        $links = $product->getLinkInstance();
        $links->setLinkTypeId(\Magento\GroupedProduct\Model\ResourceModel\Product\Link::LINK_TYPE_GROUPED);
        $collection = $links->getProductCollection()->addAttributeToSelect('*')->setFlag(
            'product_children',
            true
        )->setIsStrongMode();
        $collection->setProduct($product);
        return $collection;
    }
}

Note : Here Vendor is your module's namespace and Module is your module name in above file path.

Hope this will help you!