You can purchase a family license when you purchase the software by checking the family license option on the purchase page. The family license gives you the possibility to install the software on up to 4 computers of your household.
If you have already purchased the software with a single-user license, you can upgrade to a family license at the upgrade page. You will have to input your software registration number to access your upgrade options.
Note that the family license is only available for the ShopNCook Menu or Pro editions.