tell me a word to describe such person who often keep their silence in BBS?

Hey, teachers, could you teach me a word to describe some person who often/always say nothing in a discussion group. In another word, they are actually online but they don't want to chat with someone else. I don't know if there is an exact word for them in english. This is the first question. The second is, sometimes they have logged in that IM software, but for some reasons they don't want to make their invisible and it will makes someone else consider he/she isn't online at that time. The same question comes, how do you call them in a word in english ?(We called the second people "diver" that is translated from chinese)

By the way, there is an instant message software like MSN that is very popular in here, the above condition what I said is happen in the using of that software.

Thank s for any help in advance.