Issue to assign a seat to someone that already has an account

The principal of the company where I work, asked me to create a business account here at BOx to manage some files with a client. 

I created the account, a folder, etc, but I can’t assign to him a seat as que already has an account here at box. 
How should I proceed? 
Does he has to delete his account in order to be a part of the business account I created?