In IC a person is first set up – and they have an email address. A person is then made into a user – the username is an email address but it doesn’t have to be the same as the person’s email (but normally is to keep things simple)
You can have more than 1 person set up with the same email address, but each username must be unique.
You don’t have to validate the username so can be a dummy email address, although any alerts, approvals etc will be sent to the username.