I'll provide some details when adding an email to QuickBooks Online, MRV_Books.
Yes, you're right. You'll have to create another email address or use another email address other than the one you use for your QuickBooks account. I've confirmed this on my end and got the same error message.
You'll also want to know how to alert people through email notifications, go through this article for detailed steps: How To Turn On Work Email Notifications.
Additionally, learn how to change user permissions for firm info and client companies. Read this article for more details: Change Your Team's Admin Access In QuickBooks Online.
Fill me in if you have questions when adding an email address. I'll always be right here to help.