Just getting back to this. We use SSO to login to Discourse from the parent site, so bulk invites isn’t going to work (according to the first sentence here Sending Bulk User Invites).
What is the best approach for me? We’ve got thousands of confirmed users on the parent site, but want the ones who haven’t yet clicked though to the Discourse forum to start receiving emails. (They’re already getting discussion email though Google Groups, which we are going to switch off soon).