Creating a multi-user system to use single account using xml interface

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I do understand that this is a very generic question but I am stuck with ideas so I thought I'd try my luck in here.One of my clients is asking me to develop a website (A) that connects to another site (B) using XML. Here are the specifications:Site A registers users and keep their profile and transactionsSite B gives access to site A using a 'single' account. In other words from site B's view, site A has only one user.Site A needs to identify all transactions performed by its users uniquely and show those to its users.From where can I find some information on how to implement this? I know it is a very generic question but I simply can't any help from anywhere.