Temple membership is a slightly dubious honor. On the one hand, it signifies that your Polis trusts you to represent its interests. On the other, Temple members have a certain reputation for - well, "sleeping with the enemy" isn't quite accurate. The Temple exists specifically to make sure that the Big Three don't become enemies, and sleeping with your fellow Temple members certainly isn't required. (Although, pace the Aldhelms, there isn't any actual rule against it). But to be a Temple member is to be looked upon with a slight degree of suspicion by your own kind.
On the other hand, it's a useful job, someone has to do it, and it's certainly interesting.
As far as the Temple is concerned, you're recognized as a member if an existing member from your own Polis introduces you as one at a meeting. It's that simple. Each Polis has its own way of choosing members internally. Mage Heralds are simply Consilium officials, appointed by the ruling council and serving at their pleasure (though in practice, the council seldom replaces Heralds - it might give the other supernaturals the idea that the Mages are fractious and divided if they did). New Kindred members are proposed by the current membership at a Grand Court. The Council may veto new members, but not propose them, nor revoke existing memberships. Kindred Temple members can be removed only by a majority vote of their fellow members. Uratha members are invited to join by existing members, or may challenge the Speaker for the Temple for the right to join. The present Speaker is Van Lutyens, and no challenges have happened on his watch, which surprises no-one.
Both Kindred and Uratha have a position equivalent to "chief Temple member", called the Voice of the Temple and the Speaker for the Temple, respectively. Although treated with respect by their fellow Temple members, their only official perogative is that they report the outcome of Temple business to their own kind, at a Gathering or Grand Court.
On joining the Temple, the new member is given a briefing on its history, existing membership, current issues, and, crucially, what they should and shouldn't reveal about their own Polis to the others.
The things that the Temple members know about each other can be summarized as follows:-
They're immortal and need to drink blood.
Kindred make others like them using a process that involves draining victims of their blood, but this is difficult and never happens by accident.
Fire and sunlight hurts them. All the other traditional defences are superstitious bullshit.
They can create a special kind of human servant, which the Temple refer to as "Renfields" after the character in the novel Dracula, who are totally loyal to their Kindred masters and have a few limited special powers, including longevity.
There are different breeds of vampire called Clans, who have different types of supernatural powers. The Temple doesn't know the name of any Clan or its "speciality powers", except for the Morbus, whose special power is the ability to cause and control disease.
They have social divisions called "Covenants" which are equivalent to Tribes or Orders, but the only one the Temple knows by name is the Lancea et Sanctum (Wyncham habitually uses the old form of the name, so the Temple does, too).
The Lancea et Sanctum were a vicious, corrupt tyranny who ruled London before the Great Fire. At that time, they were dominated by the Morbus. Their basic ideology is that vampires should be evil because God created them to torment mankind and strengthen its faith.
Between the Great Fire and World War Two, used to be ruled by a Prince and Council. The Prince was killed in the Blitz, there was some kind of left-wing coup, and now there's just a Council. The London Kindred do have enemies amongst their own kind who escaped the coup, lurking outside London.
Their "town hall meetings" are held at the Natural History Museum, which is off limits to other supernaturals without prior permission (usually granted for anything that happens during the day).
Uratha are born, not made.
Their shape-shifting isn't restricted by the phases of the moon.
The reason they exist is to act as a sort of supernatural "border patrol" who prevent hostile spirit entities from crossing into the material world. Their main enemy are the Beshilu, a sort of rat-spirit with an affinity for disease and the ability to possess human hosts. All the Temple members know enough about the Beshilu to recognize their various manifestations. (The Beshilu are the spirit menace that the Temple most often helps the Uratha to deal with, so the other Temple members have always assumed that they're the main threat without the Uratha saying anything, or for the most part, even realizing the mistake has been made).
They are divided into Tribes, similar to Covenants or Orders, who specialize in different supernatural abilities. The Temple doesn't know the names of any of the Tribes or the details of their powers, with one exception. There is an outcast, renegade Tribe called the "Fire-Touched" which is the Uratha equivalent of the Lancea et Sanctum Morbus, religious fanatics with power over disease.
There is no central leadership amongst them, merely an informal alliance of individual packs. They do, however, have some unusually influential figures, like Van Lutyens.
Their "town hall meetings" are held at Highgate Cemetary, which is off-limits to any other supernatural after dark.
Their legends claim that their traditions and practices were created by the rulers of ancient Atlantis. (The Mages felt happy enough to reveal this information, since it's essentially useless in practical terms)
They are basically human, but have a gift for magic which sets them apart from the common herd. This seems to be something inborn, a quirk of genetics, like albinism.
They really can perform magic, a dizzying variety of spells which seem able to do pretty much anything imaginable. But they prefer not to use those powers casually, for some reason - possibly, the effort tires them.
They are divided into Orders, similar to Covenants and Tribes, which specialize in different types of spells. There's definitely one that's expert in necromantic magic, and another that's knowledgeable about healing magic, because their Temple representation always contains one of each type. But there are almost certainly others. The Temple doesn't know the names of the Orders or the details of their powers.
Their leadership consists of a "Prime Minister" style figure called a Hierarch - a woman, since the Mages refer to her as "she" - and a ruling council, called the Consilium. This term, confusingly, also seems to be used for the Mage body politic as a whole.
Their headquarters is in Canary Wharf Tower, which is off-limits to all other supernaturals at any tiime unless the Consilium grants specific permission.
Thomas Wyncham (Voice of the Temple)
Caspar Van Lutyens (Speaker for the Temple)
Adam Dane "Dyggvi" - Herald to the Kindred
Sam Aldhelm, "Pales" - Herald to the Uratha
Aden Brook "Munin" - Herald to the Temple
Alain Deschain "Uncle Al" - Herald to the Temple
Nick Owen "Sybarios" - Herald to the Temple