While at https://app.box.com/master/users I tried to add a staff member, but got an error about their already having an account. That could be made much better because the service could offer to convert them to an admin instead.
Anyway, I removed the staff from the Externals list, but I still get the same error when trying to add them as an admin.
It is also very surprising that I have to post about this in a public forum despite having an Enterprise account. There is no obvious way to create a ticket directly.