I was rummaging re guest access on ba.com as GGL.
Given the well reported issues of confusion on basic GGL access for domestics, you might have thought they would proof read the www site access rules:
You can also take advantage of our exclusive Chelsea lounge at New York JFK Terminal 8 when you fly with British Airways or American Airlines on a scheduled long haul flight.
Likewise, a guest may join you when you’re flying with either airline, or two guests when flying in First with British Airways or American Airways. (Passengers flying with other oneworld airlines are unable to access this lounge but can enjoy our other two lounges).
(My bolding)