You can visit any Near By Bigmart Store and ask the Cashier there to register your membership. All you need to provide is your Full Name and your Mobile Number.
Once your mobile number is registered at Bigmart Store, you can simply input the same mobile number in Bigmart App's Login Page. You will soon receive an SMS from Bigmart which contains a Smart Login Link. Tapping that link will get you logged in into the app like a Magic.
After you get logged in, you can see different promotions going on at Bigmart and you can also check all your purchase bills and savings.