This was formerly called the Ankh-Morpork Explorers' Society, until forcefully reminded by a post-imperially minded Patrician (Vetinari), that the name was a misnomer, as most of the places "explored" by Ankh-Morpork's finest and in many cases subsequently claimed for the Empire already had people living in them, from whose point of view the explorers could justifiably be called trespassers.
Any aboriginal person or other aggrieved person who wishes to set the record straight by discovering where these people come from, and claiming their society headquarters for Ghat or Tezuma or Scythia, on the grounds that the people already living there don't count, can find them at 24b Esoteric Street. Interestingly enough, several gentleman's clubs of the Fidgett's ilk may be found in the area, which makes any attempt to go around The Disc in eighty days far easier.
The current Society president is Abraham Dandy. Location currently un-known, but a flimsy raft made of balsa wood has been suggested as a part of a plan.
As mentioned in the book Unseen Academicals with further information from The Compleat Ankh-Morpork. Archive documents about border controls when entering Quirm are quoted in Mrs Bradshaw's Handbook. Here, this organisation has the alternative name of the Guild of Trespassers.