This is a bit beyond the scope of Power BI, and in general data & analytics. You will need to work with the IT/infrastructure group figure out a solution. The Report Server from company A is not recognizing folks users/emails from CorporateB probably because the server is not part of CorporateB's network.
Please work with your IT admin to discuss what the path forward is because typically in projects like this (merges & acquisitions), one company's data, users, processes etc. are migrated to another company's systems.