How do I share access to Users from another company who already have an active license?

My Company is one of 3 subsidiaries. 1 other subsidiary has active Cloud Users managing schedules. We would like to collaborate and share access to schedules when we end up working together on certain projects. I have added this company into the lists of companies in Cloud, however when I go to input User information into the office section of that company's information, I get told that they already have a license and I can' add them. How do we share access with Users with an already active license to a different company?