Hi Jacob, 

Welcome to Box Community and glad to assist! 

You're likely to get an error while adding a personal free user. This is just a reminder that you're inviting an email address with an existing Box account. 

Here's what you can do. 

• Continue to add the user, and when this error appears, "1 user failed to be added", ignore the error message and press the Save button again. Saving a second time will begin the invitation process.

For more details, please visit this helpful article. 

Thanks for posting!