When a Classlist administrator invites a parent to join Classlist using an email address, the invitee becomes a Non-Member contact, also known as an 'Invited Parent', until they go on to sign up. Although Non-Member status is really intended as a temporary phase, some parents forget to sign up or just aren't sure whether to do so. To ensure that these parents don't miss out on important messages about what's going on at their school, Classlist includes them in limited email communications (eg top level messages such as Announcements and Community Events), even though they have not signed up to use their school's Classlist site.
The system works in the same way as a mailing list: other parents cannot see a Non-Member's details on their Classlist site, nor can they get in touch with Non-Members and vice versa. Only Admin users can see that a parent has been invited as a Non-Member. This means Non-Members miss out on most of the benefits of Classlist, but are still kept in the loop.
If you have been invited to Classlist and do not wish to hear from your PTA or Class Admin via Classlist, you can click the 'opt out...' link at the bottom of the email (see image below).