There would be no point to an atheist forum.
There is no organisation for atheists. We have no creed, no catechism, no tenets of (un)belief. The only thing (the ONLY thing) we have in common is that we've worked out that there is no god. In all other repsects, and on all other topics, we hold every conceivable shade of opinion between us.
An atheist forum would simply say, "Is there a god?" to which we would answer, "No." At this point everything possible would already have been said. Anything else would either be repetitiive or irrelevant; and the forum would have to close