Alexander and Mary (-?-) Sterrett probably lived within a buggy ride from Rev. John Roan's congregation. Unfortunately, Rev. Roan served three separate congregations during this period of time: Derry, Paxtang, and Mount Joy. His congregation lists and marriage lists are in Egle's Notes and Queries of Pennsylvania.
[FYI: The Donegal Presbytery was the supervising body over many Presbyterian churches in the area. On this date, April 12-13-14, 1763, members of Presbytery (primarily ministers) met at Donegal Springs church in Lancaster Co, PA. It was common for the Presbytery to meet at a different congregation each time. The elders of the host congregation would provide housing and meals for the guest ministers.]
As for the John Fulton who came with the Sterret's, he could be the youngest son of Samuel and Elizabeth (Stewart) Fulton of Donegal Springs. He is named in his father's will (1760) but I don't know too much about him. If I'm correct, this suggests that your Sterret's were members of the Mount Joy congregation. The Sawyer and Espy names might help you identify the location.