On paper, the Migrant Merchants' Guild is a legitimate trade consortium that advocates for refugee rights and supports the Coruscant reconstruction effort. In reality, the Migrant Merchants' Guild is a highly organized crime syndicate that seized power during the riots at the old Galactic Market after the war.
Even before the war's end, the guild served as a front for money laundering and goods smuggling, but kept mostly to nonviolent crime. Its membership was primarily nonhuman, and represented a powerful economic force inside Coruscant's nonhuman immigrant population. When riots broke out, the guild saw an opportunity to claim real power; after security forces withdrew, it was the guild that re-established “order” with its armed goons and provided food and supplies… at a price.
Since then, the Migrant Merchants' Guild has expanded into bribery and extortion, with the old Galactic Market at the heart of its territory, all the while keeping a thin veneer of respectability. Republic security patrols often clash with members of the Migrant Merchants' Guild in an attempt to keep the peace, but recently the guild has been moving its influence from shaking down local businesses to Coruscant politics.