Best Practice for Attributing Employee Costs Charged to an Administrator or Assistant's Corporate Ca

We have several business units in which one person often books charges on his or her corporate card for multiple employees, which skews data on costs by employee and loses data on individual employee's cost to the organization. 

It would be an administrative burden and control risk to have that person booking the same conference for 10 people on their 10 corporate cards when he or she is not authorized to transact on those cards. 

We have thought so far of only a few ways to do this:

1. Require attendees on every expense type, the issue being free form format of typing names and titles - so it would require customization to select employees from a list and non-employees being free form as before