What IS a Thieves' Guild?
It seems to me that in these discussions that people are discussing different things when they talk of a Thieves' Guild. I wonder whether role-playing game history is at work here. I seem to recall that in the early days of D&D (I started gaming in 1981-ish), the Thieves' Guilds tended to be described as fairly monolithic. Within a given city or locale the Thieves' Guild was responsible for all organised crime. Anyone who tried to muscle in on their territory, especially the PCs, was invited to join the Guild, by paying over their dues and a cut of their takings. People who dissented were eliminated.
Of course, this model isn't too realistic to human nature, which is why some prepfer to style their Thieves' Guilds after real-life organised crime groups like the American Mafia, the Sicilian Cosa Nostra, the Chinese Tongs and the Japanese Yakuza. These groups are far tighter in membership, with only a relatively small number of people actually being part of the group. Outsiders may be 'licensed' or tolerated, according to circumstances. These groups are far harder to join, and may be highly selective over who is eligible, based on class, race, national origins or similar.
Note that I'm not saying that either way is 'right' for a D&D game. As a matter of fact I use both ways to represent different groups in different cities. I just thought that this might account for widely disparate statements regarding whether or not PCs can actually join the Guild.