1 - have that user log into the account they created. (They may have tried logging in after you deleted them and created a new account unintentionally)
2 - have them click on the Settings link and then click on the Delete button to delete the account they created
3 - now that they have deleted that company, you can add them to your account.
The next time they log in, they will log into your account.
**Note that if they have any content in that account, they should retrieve it before deleting the account because anything in that account will be deleted along with the account