As far as the average citizen knows, there is nothing to know about the beggars. There's just a lot of unfortunate people in the city. Don't be one. Those with a little knowledge of the underworld (mobsters and those people in government whose job it is to know) realise that there is more to them than that. There exists a large scale underworld organisation of beggars, and most individuals in the city who beg for a living are part of it. They have a loose chain of command, and pull together to achieve a common goal, but mostly just regulate their own profession. Anyone begging in the city for more than a few days (and hence making a permanent job of it) is approached by a beggar and invited to join their group, desist from begging, or leave the city. No-one knows exactly what happens to those who refuse all three, but the rest of the mobs have a pretty good idea. Once you've kowtowed and submitted to their authority, the Beggars seem to give their members quite a bit of freedom. If you keep an eye out you'll notice that beggars who are (so to speak) 'off-duty' can frequently be seen gossiping in pairs or groups here and there. That probably explains why information seems to flow so freely around the beggars; news seems to travel all around the city in a few hours, entirely via beggars. For the small change in your pockets, they can be quite a handy source of gossip.
In truth the Beggars are much better organised than they appear, with a system of tutors, supervisors, and captains, some of whom have invisible authority over whole districts of the city. All of these ultimately answer to the King of Beggars, a figure who has semi - mythical status to those outside of the mob. Many have heard of the King of Beggars, but most assume that he is a kind of fairy tale. No - one outside of the mob is aware who the King is or where to find him, even if they are aware such a figure exists. For example, Colonel Zero himself is aware of the King's existence, but has been unable to find out anything further. The King is the literal lieutenant of the Mutilated One in the material world, and is subject to repeated possessions by his patron. In this way, the Mutilated One exerts direct control over the Beggars. Slowly, these repeated possessions erode the King's mind, until it reaches a vegatative state. At this point a new King is chosen, with the most influential Beggars politiking between themselves, guided by dream visions from the god.
The entire Beggar organisation enjoys the patronage of the Mutilated One. Rejected by society and often damaged, many individual beggars are favorites of the god. Many more do him studious and loyal service and are rewarded by use of his powers. Officially, the enitre group regards itself as his servants. Naturally this is kept absolutely secret from outsiders; even the Emperor did not discover them in all his millenia of rule. Today, the most the Church of the Intercessor knows is that there are a higher proportion of the Mutilated One's servants in the Beggars than the general population; most of them believe that they have bigger problems than this. All beggars must make some acknowledgement and gesture of worship to the Mutilated One; however the Mutilated One is not a jealous god, and there is no obligation to become deeply involved in the faith. There is also freedom to acknowledge other gods; a favourite 'second god' is Lacrymosa.
Unlike other mobs, the Beggars are not openly associated with one particular district from which they recruit. They are based in the literal underworld of Horizon; they know the sewers of the city, and also the vast series of catacombs in which the noble dead were interred in the early days of the Empire. In these tunnels they shelter and sleep, among the sewage and the ancient dead. It is at the centre of the catacombs that the Beggar King keeps his court when he is in residence, instead of touring his kingdom. Many of the original tunnels have been expanded and connected through the labour of the Beggars themselves over the long years. In recent times they have also colonised Horizon's subway system, easily adding mapping its maintainance and ventilation shafts and adding them to their existing tunnel networks. The underground world they inhabit could be described as their 'district' but in reality they 'recruit', or at least accept members, from any and every district on the surface, and from the Four Kingdoms too. Everyone's welcome. Out of all the surface districts they have the greatest presence in the Imperial Government Ruins, although they're not overly concerned with the area, and technically the Merchants' Arm have more men with knives on the ground there.
The Beggars really do kill and dispose of those who insist on begging long term in Horizon without their authority. One significant exception to the rule are priests of the Intercessor who have taken a vow of poverty. The stated reason for this (the reason as far as other mobs are concerned) is that they have some religious feeling, and don't wish to interfere with the truly holy. The actual reason is that they can hardly be seen to be breaking the legs of dedicated priests of the Intercessor, but can't very well induct them in to the cult of the Mutilated one, either.
The initiation is more exacting than the other mobs guess, since initiates must swear fealty to the King of Beggars and to his lieutentants, and also acknowledge the divinity and majesty of the Mutilated Lord. They must pledge themselves to poverty, and if they are whole in body and mind must undergo some kind of scarring. Such ceremonies take place in the Beggars' chambers beneath the city, out of common sight. Once initiated beggars may be required to perform services at any time, but are only very rarely called upon to do so. A beggar is mostly free to live independently of his bretheren, if he wishes it. Very few do wish it, though, since within the society of the Beggars may be found friendship, support, a sense of purpose, and a connection to the Mutilated One, a god that many of the Beggars rapidly come to depend on emotionally (and in other ways; see below).
Spiritually the Beggars are the bound servants of the Mutilated One. Temporally, generations of kings have worked to establish them as the leading information brokers in the underworld. Beggars are encouraged to meet each other on a regular basis and pass on information; several of them are effectively employed by their peers to run around the city exchanging news. For a start, many beggars spend most of their day in one spot, with little to do but observe and overhear and read discarded newspapers. Beggars are seldom regarded by the rest of the population, and can get away with much more of this than anyone else could. But that's not all. They are also prepared to spend hours going through rubbish to find those bits and pieces that people throw away; many of them have become experts at reconstructing shredded documents. Not all Beggars beg full time; many more do little odd jobs that don't pay enough to support anyone in even the most meagre accomodation, and these sorts can often strike up agreements with servants to pass on gossip. If it happened in Horizon, then the chances are a Beggar knows about it [a certain amount of poetic licence taken here], and if one beggar knows, then pretty soon a lot more of them will know. In this way their captains and supervisors can put together whole tangled webs of events in Horizon. And the Beggars' information gathering doesn't end with these mundane means. The Mutilated One is generous with the power he gives out to those who follow his rule; beggars have been known to pass unseen into the houses of nobles to retrieve documents, and can speak with the vermin of the city, the rats, pigeons and other creatures, to find what they have seen on their travels, and direct them to seek out secrets (the Beggars refrain from asking Horizon's stray dogs for their news, as that would cause complications with the Loyal Order of Hounds; they are also circumspect when asking the cats for news, since Lady Illaria takes a dim view of the Mutilated One meddling overmuch with her subjects). It is even said that the King of Beggars is capable of asking the stones and timbers of Horizon for that they have seen and heard [we may or may not want to make the Beggar King this powerful]. By these supernatural (and mostly secret) means, the Beggars have gained great power over the flow of information in Horizon, more than the other mobs even begin to suspect. Meanwhile, those mobs (and others, such as journalists, government agents, etc.) come to them for news and information, and everyone pays the Beggars' reasonable prices - often in the form of another bit of gossip or a small favour.
The Beggars also have an agreement with the Ghouls, who they are slowly converting to the faith of the Mutilated One, to share information; essentially, any Beggar has the right to ask any Ghoul for news and advice, and vice versa.The two groups also trust each other more than they trust any other group in the city (which is still a lot less than completely). Unusually among humans, Beggars may (under special circumstances) be allowed to enter Ghoul lodgings, or even attend meetings of the Circle. Unusually for outsiders, Ghouls may (under similarly special circumstances) be allowed into the Beggars' tunnels, or even to the court of the King.
But that power depends in part on the Mutilated One. The current King, Seelio, is so far a dedicated follower of the Mutilated One, the god's dream visions having helped him to recover from the alcoholism that had destroyed his life and left him in the gutter. As a long - time Beggar he is well aware of the fate that befalls Beggar Kings, and is trying to prevent the degeneration of his mind. So far, he seems to be succeeding, but he has only ruled for two years. The time may come when he will look to resources beyond the Beggars to aid him. Meanwhile, many other in the Beggars spare a thought to wonder what the Mutilated One has in mind for them. Often the instructions of the God are simply advice for their own good: rescue this person from the Asylum, work against that city dignitary - but other times they seem random or meaningless, such as stealing ancient documents from the University, or speaking messages in long dead languages at very specific places. Some worry that the Mutilated one is after all just a God like any other, and is using them as pawns in games with his peers.
The Beggars have a strong fraternal tradition, and find ways to get back indirectly at those who harm their bretheren. Along with the somewhat eratic favour of the Mutilated One, this has led to a tradition in Horizon that it is unlucky to harm a beggar. By and large the Beggars get on reasonably well with all of the other mobs. It would be wrong to say that everyone trusts them, better to say that everyone trusts them not to be a threat to their interests. After all, they're just beggars.
THE UPRIGHT MEN: occasioanly even the Beggar King gets things done by breaking legs. The Upright Men are the more able bodied, tough Beggars who see to such matters. Often in action together, they are characterised by fierce camaraderie and loyalty to the King. They are also noted for the extremely good care taken of those of their members who have serious mental problems. Typically they can be found drinking in small groups at various of the very cheapest taverns when not 'on duty'. Most of them are in fact men, although there are a few female Upright Men. Some of the younger, angrier Ghouls are seen drinking with this group from time to time.
THE VETERANS: these old, broken men are the wounded remnants of the Emperor's last army. Unlike such heros as Colonel Zero, they walked or were dragged back from the front when the war ended, then sank into obscurity and poverty. They can typically be found hanging around the closed up barracks in the Imperial Government Ruins, or slowly drinking in the nearby taverns. Many of them are worshippers of the Soldier, and have connections with the cult of Sergeant Bluster that operates in the area.
THE STREET MATRONS: many children end up as Beggars. Officially no distinction is made between children and adults in the Beggars but unofficially this group of women (and a very few men) take it upon themselves to look after those who are not yet strong enough for the harsh education that the rest of the Beggars offer. The children still have to sleep in the streets and sewers, and beg for pennies, but there is at least an eye being kept on them and advice to be given.
THE SCEPTICS: these Beggars have retained most of their reason, and some degree of faith in the Intercessor. They value the protection the Beggars give them, and do lip service to the Mutilated One, but between themselves they question the wisdom of taking orders that come directly from the mouth of the god. The group's avowed purpose is to mitigate the more destructive commands given by the Beggar King when possessed; however the habit of questioning authority persists, and they also act as one of the few organised sources of scrutiny on the actions of the Beggar King at any time.