An Automated Action (this example should work with v11 and v12) can combine the followers (this example adds ONLY followers who are Users) on the Sale Order and followers from the Customer:

Code to copy/paste:

partners_who_are_users = []
users = env['res.users'].search([])
for user in users:
  partners_who_are_users.append(user.partner_id.id)

followers = []
for partner in record.partner_id.message_partner_ids.ids:
  if partner in partners_who_are_users:
    followers.append(partner)
record.message_subscribe(followers)