It has been a while since we have looked at any of the rogue organizations so lets look at The Hidden Circle, a group whom really only Dwarf and Gnome rogues will be familiar with. Lead by Hulfdan Blackbeard, a rogue trainer and quest giver they are rather more concerned with lining their own pockets than working with others for the betterment of the Alliance or even their own factions. They make their home in the basement of a house in the Forlorn Cavern which is somewhat stereotypical when you consider what they are.
The Hidden Circle represents a sort of "thieves guild" for rogues within the World of Warcraft. While SI:7 is more about spying and the Deathstalkers assassinations, the Hidden Circle are the sticky fingered bunch that are not willing to live on just a government paycheck. In fact they feel that King Bronzebeard needs to be a little more sharing in his handling of gem mining profits specifically. Since they acknowledge that the King is a busy individual, they have assumed the responsibility of distributing that wealth along lines more in keeping with their preferences by selectively removing a few pieces here and there.
When rogue players first encounter this group around level 10 they are sent off to retrieve a report from Onin MacHammer near Gnomergon. From there you just play fed ex back to Hulfdan who promptly pays you off then kicks you out. You won't be gone to long though, at level 16 Hulfdan calls you back because "King Stinkbeard" as he calls him, being well aware of the Hidden Circle's agenda, decides he has had enough and does a raid, rounding up some of our "sticky fingered" friends. At this point Hulfdan knows he needs help and you get sent off AGAIN but this time into the hands of SI:7.
Contact with the Hidden Circle other than for getting training pretty much ends at that point until you reach level 50, at that point, if you haven't gotten it already from another rogue trainer they will direct you to Ravenholdt Manor.
For role playing purposes the Hidden Circle is a Dwarf or Gnome rogues dream. A great deal of fun can be had with this specific faction. A friend of mine use them as part of the back story for his gnome rogue. The idea was that she was fascinated by the sneaky sneak aspects of rogues but her parents had run ins with the "thieve" side of the Hidden Circle and as a result declared that ALL rogues were nothing more than thieves. Because of this prejudice she had to practice her skills "on the sly" until she was able to locate a mentor whom her parent's were willing to acknowledge was NOT a thief. Another friend of mine who can not go by a humanoid without attempting to pick pocket from it had his character a part of the organization. Don't be afraid to use your imagination.