Joining:
1) Steal/Pickpocket something, get caught by a guard, go to jail, get the paper and say yes or whatever. I apologise if this is a little vague, I didn't do it this way, just heard it from someone else...
2) You talk to a begger, raise disposition to 70+ (I think?). Ask about the gray fox, be directed to gardens of dareloth at midnight (ie, make it your active quest and follow the markers). This is the way I did it.
Either way (I guess), you need to finish the mission. (incredibly easy, just remember you CAN PICKPOCKET THE LADY if she steals it first).
Making Use:
The reason I joined was in an attempt to sell the 400kgs of things that I had kleptomaniacally stolen, (*see above with regards to 5x fenced goods) thus needing a fence. The first one I found was a pretty white one with faded brown lettering across the gate, just outside Chorrel. (just kidding

). His name is Ogar the World Weary or something, and lives in Bruma.
The premise of the guild is that you steal things, fence the goods and once you have fenced enough, you can do quests for them. I am not quite sure of the amount needed to be fenced, but Greg has fenced 1600-ish and is
. He became the fox at around 1000. This means that the guild needs less than 1000 septims to be fenced in order to do all of the quests.
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