The Skinners Guilds were a variety of 'skinner' type guilds; they were devoted to killing and taking the hides of the Scriven in London, and helped spark the Scriven Uprising there. The Skinners Guilds only ever existed in London, as it was the only place to be occupied by the Scriven.
They appeared after the Scriven started to dwindle, initially in secrecy, but as the Scriven were reduced to a few hundred individuals they started to openly menace Scriven. During the Uprisings, they skinned Scriven and flew their hides like flags, hence the name.
The existence of the Guilds soon became meaningless as the Scriven were wiped out in the uprising, meaning they had no real purpose. The last surviving member of the Guilds was Bagman Creech, who was hailed as a hero in the poorer boroughs in London long after the Scriven were exterminated.
After the Scriven were driven out and supposedly extinct, the New Council of London outlawed the Guilds from having a place in the council. They were looked down upon by high Londoners, who saw them as blood thirsty thugs, and to an extent they were probably right. By the time of the events of Fever Crumb, the organisation was almost gone, and by the end of the book it ceased to exist after Bagman Creech was killed by Kit Solent.