Unfortunately the BOGO Pro doesn’t work with Grouped Products.
It is because of the way that WooCommerce handles a “Grouped Product”. WooCommerce just uses the grouping on the initial display. Once the products have been added to the cart, it is impossible to know that it came from a Grouped Product or if the products were added individually.
With that being said, there is an optional plugin called WooCommerce Product Bundles that is compatible with the BOGO Pro. This isn’t our plugin but you can find out more about it here:
This plugin will create a Bundle product that is different than the simple grouping mechanism that WooCommerce core uses.