By default Shopify do not have group products like Magento, where you can add simple products to a main product. Therefor I have made it myself on my store. I have tagged all simple product with a tag called 'free_product' and the main products with a tag called 'main'
Right now I have a problem with the cart page. Right now I have made it so the simple products quantity follow the main quantity. However this is a problem if you EX buy to different products with different quantities as you can see here: https://www.dropbox.com/s/sylmgtz2lux4vpy/group_products.jpg?dl=0
As shown in the link above then the quantites on all the simple product is 2 though one of the main products quantity is 1.
Right now I have thought about to hide the function where you can change the quantity on the cart page, so the customers only have the option to set quantity on the product page, as I then can delete the code on the cart page that tells simple products quantity should follow the main product quantity.
Any others ideas?