When a group of citizens wants to tackle a social problem online, they need to discuss the problem, possible solutions, and concrete actions. Instant messengers are a common tool used in this setting, which support free and unstructured discussion. But tackling complex social problems often calls for structured discussion. In this paper, we present Micro-NGO, a chat-based online discussion platform with built-in support for (1) the problem-solving process and (2) the action planning process. To scaffold the process, Micro-NGO adopts a question prompting strategy, which asks relevant questions to users in each stage of the problem-solving process. Users can answer the questions and vote for the best answer while they freely discuss in the chat room. For an informal evaluation, we conducted a pilot study with two groups (n=7). The participants held a discussion while collectively answering the question prompts and reached consensus to send a petition letter about campus issues to the related personnel.