The Companions Guild runs out of Braavos. All of the courtesans that work on the barges on the canals are members of the Guild. They’re extremely reminiscent of geisha, learning a variety of arts in order to entertain guests — They are not simply expensive whores. Companions are most likely to be seen as the guest of honor at a prestigious party (as the cost to hire a Companion is extremely high and it is considered a sign of status), the extremely wealthy will take them as traveling companions as they are exceptionally good company.
The Companions Guild runs and funds its own school for Companions. Wealthy families will send their prettiest daughter to go to Companion school, or a recruiter that finds a particularly clever or striking girl will cut a deal with a family in order to enroll their daughter. They are trained until they are 15, at which time they can become apprentices of a titled Companion. Once a titled Companion reaches the age of 30, she retires her title and it is passed to her most promising apprentice. Those who are not chosen become handmaidens for the new Companion until their debt to the school is paid, and those who have no debt may return home. Girls who have been through the Guild are considered much more marriageable and wealthy families will send their third, fourth, fifth, etc daughters there in order to make them more valuable in marriage pacts.
Retired Companions generally become teachers at the Guild for the rest of their lives, though there is another route: If the Companion has a client who favors his/her lady, they may put in a bid to “buy” her. She is not technically owned, as slavery is illegal, but it is more so a large donation given to the Guild to make up for their loss of a wonderful asset. The Companion then becomes a lifelong companion of their buyer, though she is not obligated to do so, etiquette wise it is expected. In some cases the Companion may marry her buyer. The Companion may refuse bids, sometimes driving up her price or driving away buyers so that she may remain at the Guild.