Online at the Rochester, New York, library was Historic Aspects Of The Phelps Gorham Treaty of July 4-8, 1788 (also see the text of the Treaty):
Massachusetts surrendered to New York all jurisdiction over the disputed territory, but gained as its reward the latter's vague title to ownership of the land. Oliver Phelps and Nathaniel Gorham paid Massachusetts one million dollars on April 1, 1788, for the lands in Western New York.
Before Phelps could reach his freshly purchased land, John Livingston and the New York Genesee Land Company negotiated a 999 year lease with the Native Americans in the area.
A Canadian company, the Niagara Genesee Land Company led by Colonel John Butler, Samuel Street and other Tory friends of the Native Americans were also interested in obtaining a lease.
Without the cooperation of the Native Americans and their friends, Oliver Phelps' deal was not easily implemented. Phelps decided to join forces with the other claimants.
The Native Americans saw the writing on the wall, and after being coaxed by a few of their confidantes, many of whom had a vested interest in the Phelps Gorham property, agreed to a treaty where a pittance was offered and accepted.
There were grievances and hopes of further negotiations (which the article further details).
From a History of Buffalo:
There were present the following persons, representing the various interests: Rev. Samuel Kirkland, agent of Massachusetts. Elisha Lee, Esq., of Boston, assistant; John Butler, Joseph Brant, and Samuel Street, of the Niagara Genesee Company; John Livingston, Caleb Benton, and Ezekiel Gilbert, of the New York Genesee Company; there were also present chiefs of the Onondagas, Cayugas, and Mohawks. James Dean, Joseph Smith, William Johnston, and Mr. Kirkland acted as interpreters. There were several officers from Fort Niagara also present. [Source]