you will need to have a new column in the users table to mark the user as being registered by CompanyA or CompanyB. Store the corresponding userID for CompanyA or CompanyB in this new column.
Your friends over here at WebAssist! These Dreamweaver extensions will assist you in building unlimited, custom websites.
These out-of-the-box solutions provide you proven, tested applications that can be up and running now. Build a store, a gallery, or a web-based email solution.