Magento – Grouped Product: Price and product list not showing on template

There is a reason the <allow_product_types> does not contain the <configurable /> product type. Magento does not have support for configurable products inside the grouped products. If you modified this, any configurable product listed in the grouped product will be treated as a simple product.
Honestly I share your interest in this, it would be a nice feature to have but it involves a lot of coding.
I haven't seen any extensions that allow this. Maybe the code 'amount' is the reason no one does this.
Maybe a good work around would be to include in the grouped product all the simple products that compose the configurable products (you may need to change the visibility of these products but I'm not sure.) Then the customer can choose what he/she needs.
Here is an example. Let's say you have 2 configurable products:

1. A couch that can be sold in black or white. (so color is the configurable attribute).
2. A coffee table sold in yellow and blue (again color is the configurable attribute)

Then you make the grouped product called Minimalist Living room that should contain your 2 configurable products, but instead it will contain 4 simple products:

1. White Couch
2. Black Couch
3. Yellow coffee table
4. Blue coffee table

Then the customer can choose a couch and a coffee table (or more) and you get the desired result.

_{I know that noone in their right mind will buy a combination like that (or would they?) but I think you get the point of what I'm saying.}

I solved this way getting the price of the last product associated (the prices of my products associated are the same for each product so I need to check what is the lowest):

<meta property="og:price:amount" content="<?php 

$product = Mage::getModel('catalog/product');
$product->load(Mage::registry('current_product')->getId());
$grouped_product_model = Mage::getModel('catalog/product_type_grouped');
$groupedParentId = $grouped_product_model->getParentIdsByChild($product->getId());
$_associatedProducts = $product->getTypeInstance(true)->getAssociatedProducts($product);

foreach($_associatedProducts as $_associatedProduct) {
    if($ogPrice = $_associatedProduct->getPrice()) {
        $ogPrice = $_associatedProduct->getPrice();
    }
}

$ogPrice = number_format((float)$ogPrice, 2, '.', '');
echo $ogPrice . " Euro";

?>" />