It is normal practice that people unrelated to the journal may be invited to review manuscripts. Are you sure the reviewer must have a Cureus email? I myself deal with maxillofacial surgery, so I'm afraid that most topics in family medicine are unfamiliar to me and will not be suitable for a reviewer.
Good day. I am interested in reviewing an article. But I am not sure of the the specific concern or issues since I am a Pediatrician and I do not have a Cereus email.