Background: https://meta.discourse.org/t/group-membership-scripting/226473/
It appears that the best way to add group membership for many users based off of their email address is using the Bulk Invite system, particularly when you have a collection of email address you want to invite to the forum and add to the group at the same time. However the already existing forum members don’t receive a notification that they are now members of the group. Sending them a message would help to prevent confusion and missed group membership.