If you're looking for a meeting place or a good stop around ½ or halfway between Atlanta and Marianna, you can try Cottage Mills, GA, which is about 2 hours south of Atlanta, or about 2 hours north of Marianna.
About ⅓ or a third of the way, you can stop at Valley, AL. This is a little over 1 hour southwest of Atlanta.
About ⅔ or two thirds of the way, you can stop at Pachitla, GA. This is about 2½ hours south of Atlanta, or about 1½ hours northeast of Marianna.
About ¾ or three fourths of the way, you can stop at Ferrell Crossroads, GA. This is almost 3 hours south of Atlanta, or about 1 hour north of Marianna.
If you drive straight through without stopping at all, the total distance is 249 miles. The distances shown above don't include local roads to get into each town, so if you're planning a long road trip, make sure you factor in traffic and road conditions, rest stop breaks, and interesting detours along the way. If you're in a rush, you can do the trip with zero stops, and your total travel time would be around 3 hours, 56 minutes.