Product Grouped can addable the products other on a product page.
To create a product page and be able to display more such other products. You need to follow these steps:
Open a product page available, copy name of that product.
Go to Online Store -> Navigation select Group Menu.
You need enter fill these sections: Title, Handle & Menu Item, then click Save.
Go to Products -> Choose product just have added -> CLick to View To see the result:
Note: When you use Product Grouped features, you can only add cart for group products which you already setup from navigation, that mean add cart button for main product page which you are seeing not show.