How do I add an external user

Hi Chris, 

Welcome to Box Community and glad to assist! 

Adding External Users is as simple as inviting collaborators. With Box, you can invite collaborators from outside your company into folders you own. These users, known as External Users, can be given the same permission levels and have access to the same Box features as your Managed Users. Here is a quick overview on the differences between managed and external user abilities: 

Best Practive- Managing External Users

Thanks for posting!