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1. find function renderForm()

2. add before 'type' => 'group_discount_category

[
                    'type' => 'switch',
                    'label' => $this->trans('Show prices', [], 'Admin.Shopparameters.Feature'),
                    'name' => 'show_prices',
                    'required' => false,
                    'class' => 't',
                    'is_bool' => true,
                    'values' => [
                        [
                            'id' => 'show_prices_on',
                            'value' => 1,
                            'label' => $this->trans('Yes', [], 'Admin.Global'),
                        ],
                        [
                            'id' => 'show_prices_off',
                            'value' => 0,
                            'label' => $this->trans('No', [], 'Admin.Global'),
                        ],
                    ],
                    'hint' => $this->trans('Customers in this group can view prices.', [], 'Admin.Shopparameters.Help'),
                    'desc' => $this->trans('Need to hide prices for all groups? Save time, enable catalog mode in Product Settings instead.', [], 'Admin.Shopparameters.Help'),
                ],
                [
                    'type' => 'switch',
                    'label' => $this->trans('Assign to all categories', [], 'Admin.Shopparameters.Feature'),
                    'name' => 'all_cat',
                    'required' => false,
                    'class' => 't',
                    'is_bool' => true,
                    'values' => [
                        [
                            'id' => 'all_cat_on',
                            'value' => 1,
                            'label' => $this->trans('Yes', [], 'Admin.Global'),
                        ],
                        [
                            'id' => 'all_cat_off',
                            'value' => 0,
                            'label' => $this->trans('No', [], 'Admin.Global'),
                        ],
                    ],
                    'hint' => $this->trans('Customers in this group can access to all categories. Leave on "NO" if you want to assign them manually.', [], 'Admin.Shopparameters.Help'),
                ],

3. find function processSave()

4. add after $this->updateRestrictions();

5. create new function after processSave()

public function access_all($group)
    {
        $id_group = (int)$group->id;
        $assign_to_all = (bool)Tools::getValue('all_cat');
        $get_categories = Db::getInstance()->executeS('SELECT id_category FROM '._DB_PREFIX_.'category WHERE id_category > 1;');

        if ($assign_to_all && $id_group && $get_categories) {
            foreach ($get_categories as $category){
                Db::getInstance()->insert('category_group', array(
                    'id_category' => (int)$category['id_category'],
                    'group'      => (int)$id_group
                ),false, true, Db::INSERT_IGNORE);
            }
        }
    }