Adding existing user to enterprise

Hi Stephanie, 

Welcome to the Box Community!

Below are the steps on how to add a user that has already created a (free) Box account:

1. Go to Admin Console > Users and Groups.
2. Click the Managed Users tab.
3. Click Add Users () and then click Add Users Manually.
4. Enter the managed user's name and email address. 
5. Click "save"

(Note: Please disregard the message "Email already used in a Box account but not associated with an enterprise" just click save option twice (at first click, you'll get a failed error, then second time clicking the 'save' option will give you a prompt that says "1 user added" or "invitation was sent to the user")

I trust this information proves helpful. Thank you for posting!