Setting up private multi-stores

Hello, I am trying to set up a website for an associate of mine. I hope that this is the correct forum to ask.

He to be able to easily setup multiple, private storefronts where clients are able to buy products made specifically for them. He wants clients to be able to access these stores with their own unique password which can be entered on the main page of the website, which redirects them to their store.

For example:
John Smith is given a code JS123 which he can enter on the main page to redirect him to a store that sells the t-shirts our company makes for him. Ann Thomas is given the code AT321 which redirects her to the store that sells hats we make for her. Neither will be able to see the other’s items without the password.

What would be the best way to accomplish this?