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You seem to misunderstand Usenet. To simply reply, just do it. No special action is needed. The reply goes where it was told. And 'joining other groups' is trivial - bring up the 'subscribe' page, search for the group name (only one, staring you in the face), tick something (depends on reader), done. This is only needed if you plan to follow the discussion.
This assumes you have a semi-reasonable news-reader.