In a situation of articles and comments, a comment is "posted by" someone. How to define the person who posted the comment?
one who comments; especially : one who leaves a comment on an Internet site, story, page, etc.
Also note (all from Merriam-Webster):
Questioner (under question):
to ask questions : INQUIRE
— questioner noun
Answerer (under answer):
1 : to speak or write in reply · neglected to answer the letter
— answerer noun