Issue
As a Manager for a Google Drive folder, neither I nor the other managers are currently able to add other users to some of the subfolders that we are also managers for as Content Managers.
Environment
Google Shared Drive.
Resolution
Add the external email account as a Contributor, not a Content Manager. Email accounts that are not within a Google domain can only be Contributors, which will allow them to edit content within the folder.
Cause
External Account is not in a Google Domain.