I would suggest: make an Announcements category (or Polls, or whatever is appropriate) and make the site default to be Watching that category. Or at least Watching First Post. Then, they’ll get notified when there’s something new in that category.
People will be able to opt out, but you could explain nicely in a sticky post why you’d like them not to. And really, they can always opt out by leaving the site, so… if someone prefers to not get the default-global notifications… probably ok?