It would be best to contact the person running it. It's not like a facebook group that you add, but there are forum threads that everyone meets in on the designated date and talks about (or at least tries to stay on subject about) the product that the meeting was for. If the porn club works like the Erotica club, once a month you get to pick one of the pornography shows the discussion is going to be about, and it gets assigned to you to review. That way you get to review a free item AND make a bunch of friends!

You also can just show up on the designated date and time (don't forget about time zones!) and ask how you should go about joining.

I hope I helped!