Hi Bradley, 

Welcome to Box Community!

Kindly disregard the message "Email already used in a Box account but not associated with an enterprise". You just need to click the save option twice. (After your first click, a red banner will appear that indicates failure. Press the save button once more and you'll get the prompt that says "1 user added". Then, the user will receive an email invitation to join the paid account.)

Here's the article for reference https://support.box.com/hc/en-us/articles/360044195233--1-user-failed-to-be-added-Message-When-Adding-Managed-User

I hope this answers your question. Thanks for posting!

Best,
Ann