There are two different email types that can be added to a candidate's dashboard:
Their contact email - which is the email that the Mail Center system uses to send invites.
Their Campaign Email - which is a public email address that is displayed on the front end.
While both fields can contain the same email address, in order to send the candidate an invite, the email address must be in the contact email field. In the image below, this is located at the top of the candidate's race profile (indicated by a green square). In order to add or change this email field, you must first click on "Edit Candidate Information" (indicated by a green oval) to access that field.