In the Smart Send system there can only be one owner of a website and the subscriptions associated with that website.
To transfer a website to another user simply login to your own Smart Send account and change your e-mail to the new user.
The new user can then use your old password or can reset it when trying to login to the system with the new mail.
If you get the error Email already taken, you need to login to the user associated with the mail your are trying to transfer the owner ship to and delete the account in the profile tab.
Note that deletions are permanent and deleted accounts or there subscriptions cannot be recovered.