Hi there, 

Welcome to Box Community and glad to assist!

Let me create a new case and a member of our team will be in touch, please keep an eye out.

Thanks for posting!

I too have received the same error message when trying to add a user.

Was this resolved? I am having the same issue.

I am having the same issue. The person I am trying to add had a personal Box account with that email address, but he deleted the personal account. I still cannot add him and it says his email is already associated with a personal Box account.