The old saying is that there is honor among thieves. And this is true, to the extent that if you're a member of the Thieves' Guild, the other members will follow the Guild's Code in respect to you, and you will extend them the same courtesy. Honor to common pickpockets or marks? Never.
The Thieves' Guild is an organization that most law enforcement would dearly love to get their hands on...if they could be sure it exists. Some write it off as a rumor, a myth perpetuated by criminals to make them seem better than the scum they are. Some swear it exists, but it's damn well hidden. Most decent, law-abiding, normal citizens have never even heard of such a thing.
Well, it does exist. Members of the Guild pay dues to the organization to help keep it running, and in return can expect hospitality (and hiding) at Guild safehouses, help for their families, even legal aid should they get caught (or a simple jailhouse break-out, depending on how hot the water you're in is). The Theives' Guild helps set up dependable fences, sending proceeds back to your family or to a secret account somewhere, depending on your written wishes filed with the Guild (and after taking their share).
There is a Guild Headquarters, of course. There are several, and they are excellently well hidden. The largest Thieves' Guild HQ is in DiCi, of course, but there are branch headquarters and safehouses in Rogue's Cove, Northport, Darak Nil, and Qua'Nath. There's even one in Clawmoore, the only safehouse and headquarters known of by the local authorities. This is a little riskier to use, since the Dragon authorities operate on an open system: if someone comes and asks specifically after a local HQ and/or safehouse of the Thieves' Guild, the Dragons will tell them exactly where it is, no matter who they are. However, only members of the Guild know that one exists in Clawmoore, and they already know its location. The very idea that the Dragons would knowingly allow the Thieves' Guild to operate in their only city has never crossed the mind of any bounty hunter or law enforcement officer, whether they believe in the existence of the Guild or not. The Thieves' Guild has this sweet deal thanks to a solemn pledge to never steal from any Dragons while in the city. The last time a thief picked a dragon's pocket in Clawmoore, the Thieves' Guild killed him themselves.
In the old days, membership was as simple as stealing something impressive enough. After a bad scrape with the law caused by a law enforcement mole, membership has become much more complicated.
First off, there's no way in without the sponsorship of an existing Guild member. Your sponsor stands up for you, looks after you, and leads you through intiation and your first few months in the organization. Some thieves even take on proteges, naturally showing them the ropes of good thieving as well as undertaking to get them into the Guild.
You must swear to uphold the Thieves' Code (as listed below). There are some thieves who know the Code and hold to it even though they aren't in the Guild, but this usually means that they want to be in the Guild (and can be a sign of faith to any would-be sponsors). Breaking with the Code will get you immediately expelled from the Guild, as well as putting a black mark on your sponsor.
That's it, aside from finding a Thieves' Guild member and/or the headquarters in the first place. It doesn't sound like much, but given the rigorous Code you must hold to and the extreme difficulty of finding a sponsor, given the fact that Guild members do not need to extend the Code to outsiders (and indeed, often consider it foolish to do so), well...sometimes going straight almost seems easier.
Remember that the Code is only applicable to other members of the Thieves' Guild. Non-members and marks (any one not a thief is a "mark" - a target for pickpocketing or thieving) are not protected under this Code, even if they claim to hold to it. You may choose to extend the Code to them if you so choose: for example, a non-member who does hold to Code and seeks membership, or a member of your family. However, breaking Code with them incurs no penalty. Breaking Code with a fellow member of the Guild means immediate expulsion from the Guild - at a minimum. Depending on the severity of your crime, you may find your soul expelled from your body as well.
You swear to keep all vows, oaths, and promises to fellow Guild members.
You swear to keep secret and hidden, upon pain of death, all Thieves' Guild headquarters, safehouses, and other necessary structures.
You swear never to reveal, upon pain of death, the officers or other members of the Thieves' Guild.
You swear to pay 25% of all scores and fences to the Guild as payment for its services and protection.
You swear to never reveal to any outsider that you are a member of the Thieves' Guild, or that it exists, unless you are sponsoring a thief for membership.
You swear never to steal from another member, his or her family, or a Dragon (in Clawmoore city limits). Taking scores from another member is grounds for expulsion.
You swear to extend hospitality and safe hiding to all other Thieves' Guild members in need of it.
Hospitality and safe hiding from any Thieves' Guild member should you be in need.
Aid in escaping from jail or in court.
Reliable fences with good prices should your own fences be arrested/killed or go underground.
A secure way to send funds, items, letters, or other packages to family, loved ones, or other persons or accounts you should need to send it to.