For anybody else wondering:
I could never get my head around it either, it's not really properly explained anywhere that SA is a separate site the joining and subscribing to which, at about £60 a month, will let you, not only use the other site and message board there, but also have access to the SA thread on this site.
I never joined, it seemed a bit steep basically just for a look to see if it could be useful, does anybody have much experience of using it?