How do we define left? Well this is rather simple: do they support single payer MedicareForAll? Do they support things like the Green New Deal and environmental sustainability? Here you will find a list of public organizations that support these policies.
National (currently) unaffiliated orgs:
https://twitter.com/iww <<< These folks are an OG union and helped give us the 8 hour work day and helped end child labor. Always support the IWW.
https://twitter.com/DemSocialists <<< Democratic Socialists of America. Join it if you can afford it. Lots of folks have gripes about them, but they're the best we got.
https://twitter.com/dsa_lsc <<< DSA Libertarian Socialist Caucus. We're huge fans, obviously.
https://projectsouth.org/ <<< Left legal & political advocacy
http://foodnotbombs.net/ <<< National Food Not Bombs
Local (currently) unaffiliated orgs:
https://twitter.com/iwwatl <<< Workplace doesn't have a union yet? Get in contact with these folks and organize with them. Have a union? Do that anyways. More unions, more good.
https://twitter.com/northgasra <<< For Socialist folks in North GA that like guns and want to defend themselves.
https://twitter.com/ydsagsu <<< For GSU Students
https://twitter.com/AthensYdsa <<< For UGA Students
https://twitter.com/YDSAGT <<< For GaTech Students
https://twitter.com/ydsa_ksu <<< For KSU Students
https://twitter.com/ydsaemory <<< For Emory Students
https://twitter.com/GCYDSA <<< For Georgia College Students
https://twitter.com/metroatldsa <<< General Atlanta DSA
https://twitter.com/DSA_AugustaCSRA <<< Augusta DSA
https://twitter.com/SCofNorthGA/ <<< Former North Georgia DSA
https://www.facebook.com/SunriseATL/ <<< Sunrise Movement, Atlanta
https://twitter.com/sunrisenga <<< Sunrise Movement, North Georgia
https://twitter.com/OWSAtlanta <<< Occupy Wall Street Atlanta
https://www.facebook.com/SavannahFNB <<< Savannah Food Not Bombs
https://foodnotbombsatl.org/ <<< Atlanta Food Not Bombs
https://twitter.com/GLAHR_ <<< Spanish speaking Abolish ICE Movement
https://twitter.com/BAJItweet <<< Black Alliance for Just Immigration
https://twitter.com/ignitekindred <<< Southerners On New Ground, GSRM Organizing, intersectionality