The way I see it is like this: Writer's Forum = Writer is (or has or any other verb ending with an s) Forum Writers' Forum = This forum belongs to someone/thing called Writer So I personally think this should be called Writers Forum = This forum is for anyone who is a writer :) However, being new to this sort of thinking I could be wrong, but I do try to get my information right before I sprout off but I still could be wrong lol