If you are trying to add a user to your account but that person already signed up and created their own separate account, you will not be able to merge both accounts together.
To fix this, and add their email to a new user in your firm, watch this video or follow the steps below:
1) They must navigate to the "Settings" page.
2) Change their account email address by adding +1 to the end of the local part, mailbox title, of their email address. e.g: change email@example.com to firstname.lastname@example.org.
3) Click "Save".
What happens now?
Now you will be able to create a new user to your firm's account using their original email address!