"Email address is already registered with another account" How to fix this?

"Email address is already registered with another account" How to fix this?

Please help!
When I try to create a user account for  missbrittany@morningside-montessori.com , it gets put in the "Invitation" list. If I try to make  missbrittany@morningside-montessori.com  an alias for an account, I get the error: The email address is already registered with another account which is not a part of this organization.

What do I need to do to create a user account for  missbrittany@morningside-montessori.com ?