Magento Create Seperate Logins For Different Stores

Hi Guys,

 I needed some help with logins for different stores. Let me explain the project requirement. 
  1. There’s a main website which has links to three different stores. Let’s consider the 3 stores are Nike, Reebok And Puma.

  2. The admin creates unique users for each store, i.e the reebok store will have a unique user, the nike store will have a unique user and so on.

  3. The reebok customer , once logged in, can checkout items from the reebok store only and cannot checkout items from other stores.

  4. So basically, this means that customers assigned to specific stores can checkout items from only their respective stores and not other stores.

           Could someone please point me in the right direction to solving this requirement.