First, follow this article to ensure that the user you are looking to transfer ownership to has been added to your account.
Once the new "user" has been successfully added to the account, you can transfer the ownership to this email user by following these steps:
1. Log in using the account and email with owner permissions.
2. Click on the Settings button in the lower left-hand corner of Tend
3. In Settings click the "People" tab
3. Click "Edit" on the "Owners" account
4. Click the "Transfer ownership to another person" drop-down and select the new account's user name and save.