Where I can find this option?From Tutorial:
Sharing products and categories
When you create a new store within a group, you can choose to have all or some of the categories in the new store be exact duplicates of the categories in any other store on your installation of PrestaShop.
When creating a category, either for a specific store or for all the stores in the PrestaShop installation, PrestaShop registers the category for all the stores – it is simply hidden from any store where it has not been set.
By associating the new stores with a given category, any change in this category will impact all the stores which are associated with it, even if the stores are from different store groups. You can therefore change the category's content once and for all from one place, including its products.
But if seller from secondary shop add category or products in main story I didn't see that.
But when I instal shop group I set Share customers, Share available quantities to sell and Share orders. Once enabled.
Maybe I miss something?