You asked how I will account for lurkers on the Teaching Sociology newsgroup. Yes, I know that lurkers are present in unknown numbers. But if they do not post messages to the group, I cannot account for them. How could I? Lurkers present no behavior or communication on the newsgroup and hence cannot be observed.
You asked how I can contextualize "living" with the group if I do not teach sociology. My assignment was to observe a newsgroup with professional interests, such as newsgroups for musicians, dentists, photographers, et cetera. I'm "living" with the group because that is the very premise of fieldwork. No, I do not teach sociology but I am "living" with a group of sociology teachers for fieldwork purposes.