Looks like you could craft a URL for a group's preview page that lets you see latest messages without joining the group, and by periodically scraping that page you could basically spy on the group.
Finding group IDs could presumably be easier because they could be passed around more easily if members thought no one can read anything until they are approved to join.
Finding group IDs could presumably be easier because they could be passed around more easily if members thought no one can read anything until they are approved to join.