How are you doing? --Fine. How are you doing? --Just fine. In this context, the word "just" makes the answer "fine" a little bit stronger. It is similar to "really" in these greetings: It is wonderful to meet you. and It is just wonderful to meet you. = It is really wonderful to meet you. The just or really usually expresses a stronger emotion on the part of the speaker. Or it could just be politeness. In reality, the greeting with or without the "just" is practically identical and not something a beginner needs to worry about. If you are unsure whether to use "just," I suggest you don't use it.