It seems surprising to me that this isn't already something Workfront is capable of, however after talking with support, it doesn't seem currently possible. The idea is that if two separate companies (actual companies, not companies within your WF instance) both utilize Workfront and have their employees setup in it, to be able to allow users from one company to have access as users in the other companies instance.
As a real world example, if we as company A have a contract to get work done with company B, where people at company B would be responsible for certain tasks in our project workflow, it seems logical that if company B already uses WF for their own internal needs and users have accounts, that we would be able to add certain users from company B into our instance of WF at company A since they are already licensed/etc.
Since this seems to not be possible, we at company A need to pay for licenses (or have company B pay of them) in order for those employees at company B to be users in our (company A's) WF environment. Seems like WF is now getting paid twice for the same user, just because they are in two different instances.