Here's how it works in more depth:
You (the leecher) find somebody experienced at DG to suicide-key. This means that they will not worry about their own xp and try and complete the dungeon as fast as possible for your own xp/hr sake. They either recruit 3 more random people from w67 (which is the most common) or already have a selling team organized. Then you pay the sui-keyer, and they do a floor while you mostly AFK until it's done. Do not go completely AFK because you will be needed for skill doors that you can do and no one else, for levers/emote/portal doors, and to end the dungeon.
I don't know the good FC's, but I think "Leech Dream," "Keyer," and "Sell Dung" are the biggest (in no particular order). A good level to start leeching is at 59 when you can at least do floor 30. Buy f30 then rush/solo 1-29 and repeat. 99% of all the sui-keyers will only sell floors 30-60, but if you are really desperate for the fastest xp possible at <59 you may be able to find people to sell you Frozen, Aba1, and Furn floors.