If you are staying in the city before a flight, these are the options we found.
Shared-ride shuttles
ATL lists maximum fares per person: Downtown US$16.50, Midtown US$18.50 and Buckhead US$30. They collect from Aisle B in the Ground Transportation Center. From the International Terminal service is limited, so phone the company first.
MARTA
MARTA trains serve the airport. We have not checked fares or times, so look them up before you travel.
Staying near the airport instead
If your flight is early, a SkyTrain hotel saves the trip into town. See hotels connected to the ATL SkyTrain.