Fair enough 
This command will turn on the “Activity Summary” (referred to as “digests” in the source code) for everyone, set the frequency to daily (1440 minutes), and include posts by new users:
User.all.each { |u| uo = u.user_option ; uo.email_digests = true; uo.digest_after_minutes = 1440; uo.include_tl0_in_digests = true; uo.save! }
Edit: I didn’t bother to handle the case where people have daily emails set up, as this will (eventually) be taken care of by the migration: https://github.com/discourse/discourse/blob/0984763013bc18f62912d55bd3c12a83423b1b83/db/migrate/20170505035229_migrate_mailing_list_daily_updates_users_to_daily_summary.rb