Cumming is about 40 miles north of downtown Atlanta near Lake Lanier. It is MARTA accessible thanks to a connection with a regional bus service, GRTA Xpress route 400. The bus service is limited and only runs Monday-Friday to/from the Cumming Park and Ride just off GA 400 exit 14 at 163 North Old Atlanta Road, Cumming, 30040.
As far as connections with MARTA, the bus departs from the North Springs and North Avenue MARTA stations (North Springs is the closest station to Cumming). There are additional departure/drop off locations within two blocks of the Five Points and Peachtree Center stations.
GRTA Xpress fares are cash only or can be deducted from a MARTA Breeze card. It is more expensive to travel from downtown to Cumming ($4 one way). It costs $3 one way to travel from North Springs to Cumming.
Uber is another good option. Request a car at the North Springs station and use the promo code MARTAGuide to get a free first ride up to $20.