https://github.com/wikimedia/mediawiki-extensions-Newsletter/blob/master/includes/NewsletterDb.php#L95 should accept UserIdList just like addSubscription above. Same for RemovePublishers.
Possibly affected code would be:
- https://github.com/wikimedia/mediawiki-extensions-Newsletter/blob/master/includes/NewsletterEditPage.php#L415 -- here we should directly call our addSubscription with the whole list of users instead.
- Below that, the notifyPublishers() should also take a userIdList and this should happen in the next line.
- https://github.com/wikimedia/mediawiki-extensions-Newsletter/blob/master/includes/content/NewsletterDataUpdate.php#L126 -- same here.
- Probably the removePublishers() too.
Difficulty: Moderately tough, but easy once you read the examples given above.