Depending on what they mean by "visual features", this could be a real stretch. You might give serious thought to this offer having a low probability of payoff for 3 weeks of work, and consider whether you want to risk that amount of time. The systems I know that let people chat (via web pages) all required several person months to get to a stable state.

If you decide to go for it, merlyn has aWebTechniques article that describes a simple web-based chat system.