The Guild is a society of semti who use their powers to fight against followers of the Nameless (and thereby against their master). It is not exactly secret; just about everybody knows that it exists but they are not really interested, it's outside their sphere of attention.
The Guild is structured like a craft guild with apprentices, journeymen and masters, except for the fact that most people have their ordinary job as well. In fact the only people who work for the Guild for a living are the Sworn of Anshen, Guild runners and the teachers of the Guild school in Turenay. The Guild itself has no political power, though the Order of the Sworn has some political influence.
The only qualifications necessary for entrance are ability and motivation, and find someone to support you or have someone find you. It's like the St. Something's Guilds in the Middle Ages where people from all walks of life banded together to pursue a common cause. It's a calling, you're not in it for prestige because it's semi-secret anyway, and once you're in you have to work very hard for very little return except the gratification that you did a good thing.
There are no entry fees, only a yearly contribution ranging from nothing if you're penniless to a hefty fee if you're rolling in money. It's mandatory to contribute something - even if you can only give some of your time to do leg-work, or some of your brain-power giving lessons - but how much is a matter of conscience.
Exams usually take the form of a trial/initiation. Though you have to learn, you can't study specifically for it: most likely it will catch you by surprise when you're ready.
A Guild runner is basically a secret agent in the service of Anshen. It's a difficult and dangerous job with a high mortality rate: more than half of all those who start out as runners and don't quit, die within five years. Let's be charitable and call it a vocation. For some it's an addiction.
Part of the work of a Guild runner is to look out for new talent. If you have the necessary ability and start to show it (tends to come with puberty, but high-ability people are often earlier) you're likely to be 'picked up' by somebody, even if nobody in your family is in the Guild.