A join project between Microsoft and American Express, code named "Rome", allows companies to ____.

The joint project "Rome" between Microsoft and American Express focuses on providing companies with the ability to have greater oversight and control over their business travel expenditures. This initiative allows organizations to require their employees to purchase their own business travel tickets through American Express, effectively streamlining the purchasing process and ensuring more consistent adherence to company travel policies.

By mandating that travelers book through a specific channel, companies can better manage expenses, track travel patterns, and potentially negotiate better rates with suppliers based on usage data. This option also helps to facilitate compliance with corporate travel policies, as the process of booking and approval can be integrated within the system.

In the context of the other options, planning group travel, booking hotels exclusively, and managing corporate events may be significant aspects of travel management, but they do not encapsulate the core offering of the "Rome" project, which specifically emphasizes rigorous control over the purchasing behavior of corporate travelers. Therefore, the correct choice reflects the primary intent of the partnership's service, making it applicable and purpose-driven in the realm of corporate travel management.