You can just walk up to them and talk, but since they are sitting in one of these chairs, they likely will want a brief conversation and be left alone. Also, socializing in other places like a bar or in a park is not outlawed. Do try to be friendly and courteous. The best way to start a conversation is by introducing yourself. Then, hop into what you wanted to talk about. Maybe even try to start with a topic that can transition to your final goal. In the end, there is an intention behind every social interaction, and it can either be casual or formal. Maybe that is what you are desiring, for a known intention/goal to connect with others.