How about this:

By default, a user sees all Meetup announcements and other announcements of regional interests.

A user may edit their settings to turn hide regional announcements except those for regions they care about.

Regional announcements will include a link the the relevant settings page.

One potential issue is that if a region is added, interested users might not find out about if they have already opted out. If regions are added slowly, it could work to show users new regions by default even if they have opted out before.