Hi Stephanie
I send an email to all Year 12s in Term 3 welcoming them to our past students association. The email contains a link to a google form where we collect their email address (postal address is rolled over from student records) and get them to indicate what areas of the school they are interested in hearing about. I usually have to send a reminder to those that don’t reply.
When I am organising year group reunions I include a list of alumni from that cohort that we don’t have details for and ask that the invitation email is forwarded to them.