Click the "Explore your morality" link towards the bottom of the linked page.
That just takes me to a list of studies I've taken and that it suggests I take. There's a short paragraph on how to form a group myself, but nothing on how to join one.
Re-reading the blog post, it seems that there's a separate link required for those who were already registered. Is that correct, and do you have it?
Here's the news article on this: http://www.yourmorals.org/blog/2011/11/how-to-use-groups-at-yourmorals-org/
And here's the group that the LW community just created: http://www.yourmorals.org/setgraphgroup.php?grp=623d5410f705f6a1f92c83565a3cfffc
I think it will be very interesting to see what we can all get on this.