There is no *A lounge in PEK.
You can use a lounge if you are *G.
They will give you an invitation card at check-in for either the Air China lounge or the BJS contract lounge.
Both suck.
The general consensus is that the BJS lounge may suck less than the Air China lounge.
Additional consensus is to ignore both lounges and head to the Starbucks in the departures area. It apparently sucks less than both lounges.
Or else arrive at the airport at the absolute last minute. This apparently is the least suckiest of all options.