You switch on Destiny, and open your friends list. You scroll a tab, and are looking at one of the private groups you have joined on bungie.net. There is a files feed and browser, with pinned top files (as assigned by group admins). There is also a list of games. These are called "group matches". In making the game lobby the host chose the option "open to x private group" (instead of "open to friends"). As result the game appeared in the specified group games feed.