{An idea I came up with when fleshing out one of my 'iconic' characters Frelassa Lucette (her rough stats are [NG, Female Yugoloth Tiefling, Bard/Sublime Chord]) is that she becomes to an organization known as the Ishtaritu.}
Overview:
Part guild and part secret society the Ishtaritu have existed for eons on Sigil, Elysium and many other places across the planes.
Now the Ishtaritu is the actual name of the priesthood or sacred prostitutes of the goddess Ishtar. They aren't exclusively a religious sect to Ishtar, as many of their members are non-religious or revere other goddesses such as Freya, Aphrodite and the like. Their mostly matriarchal, but they aren't exclusively female; it's just that most of their members are female. They are also a support and advocacy group for prostitutes.
While they do have an agenda of their own, they aren't exactly a sect of faction, but their members to lend to lean towards the Society of Sensation and Revolutionary League. They're against the any of the taboos and prejudices held against sexuality and gender (such as prudishness, sexism and homophobia), and are also against any of the perversions of sexuality (such as rape and pedophilia).
{In some ways they do resemble (and are inspired by the) the Companions of Firefly/Serenity, as well as the Bene Gesserit & Honored Matres of Dune.}
Membership and Initiation:
The Ishtaritu are a society that does exist out in the open, but they have many of their own secrets. First of all they don't take anyone in as one of their members; one has to be chosen by the Ishtaritu to become one of them. What they look for in their members is that they are insightful, intelligent, sensual, compassionate, open minded, self-sufficient and show some sort of potential to bring something that adds to the guild. Many members of the Ishtaritu were common prostitutes before joining, but not all of them are, and they can in fact come from many varied backgrounds.
If their prospective member accepts entry into the guild, they become an initiate. Initiates are taught many of the secrets of the guild, such as etiquette, art, psychology, rituals, martial arts, magic, psionics, and more. They are trained in the arts of seduction, conversation, and of passions. Initiates when training in the practice of sex often do with each other, and with their instructors, as they aren't prepared to take on clients of the guild just yet.
Eventually they become apprentices, and do entertain clients that the guild has set them up with. Apprentices don't necessarily have sex with all the clients they have, as it isn't the point of their training. Mainly they are learning and getting experienced when with these clients, and they are being monitored at this stage by their mentors.
After a few years, apprentices are ready to become full disciples of the Ishtaritu, where they can take on clients by themselves and strike out on their own if they desire.
The Ishtaritu has a wide collection of lore, including a secret history which they keep from all outsiders. They have a large number of rituals (Incantations from Unearthed Arcana) and spells and techniques among their secret knowledge. Their secret lore and prophecies speaks of an idealized and transcendental age to come, in which they can bring about by manipulating people towards their goal. They don't seek to rule above all, but they intend to be the architects of this new age to come.
I like this idea, it's interesting - although your use of society and guild makes it confusing as to what their structure is.
In the 7th Sea camppaign setting (AEG, D20 version is called Swashbuckling Adventures) there is a secret society called Sophia's Daughters that has a similar setup to your description.
What are the long term goals of the Ishtaritu?
Do they have any dirty secrets that would cause a major upset if it became known amongst the majorit of their members?
What are their views of the current Sects and Factions?
Are they aiming to create a seamstresses guild in the post-Faction War Sigil?