The Association of British Commuters said it planned "to hold the government to account" with a judicial review into the Department for Transport.
Services have been disrupted for months amid a long-running dispute over the role of conductors.
The Department for Transport declined to comment.
Last week the government unveiled a £20m fund in a bid to improve services and "get to grips" with problems on the network.
RMT union members are due to stage a 48-hour strike on Wednesday and Thursday in their fight against changes to the way services are operated, including giving drivers responsibility for opening and closing carriage doors.
The company began imposing the changes in August despite union concerns over safety and job cuts.
Southern said more than two out of five trains would be cancelled, with some routes having a limited service or "no service at all".
Passenger services director Alex Foulds said: "This two-day strike will achieve nothing.
"After many months of trying to reach agreement with the RMT, we are now moving forward with our plans for the benefit of customers and we urge the RMT to join us in putting passengers first."
RMT general secretary Mick Cash said the union remained available "for serious talks".
The Association of British Commuters said it would be working closely with lawyers to compile evidence of people losing their jobs because of cancelled trains, as well as any disruption to home and family life.
It has launched an appeal to raise money to fund the legal action, and hopes to reach a target of £25,000 within the next 30 days.
Spokeswoman Summer Dean said: "We believe that the government has been very quiet during this crisis and we are now ready to use the law to demand answers."
A rail passengers group is to fund a legal review of the government's handling of the Southern rail franchise amid continuing disruption to services.