From what I understand it's a little bit of what you suggested, and a lot of automation. Many fansub groups use specific trackers and list their torrents in a centralized place. The system checks some of these places and adds torrents to the site listing as it finds them. In cases where there's an unknown fansubbing group or somewhat random torrents they are added manually.
I could be wrong, of course
(I'm pretty sure this has been discussed before but I'm not sure which thread it was in.)