Just over on the left of the screen, in the "Around the Site" box you'll see Join a Group. Click on that and it will take you to a new page with a list of groups you can join - that includes the SU Demo group. Just click on the Join Group for that option
.
I'm not sure whether you get notified when you've been approved for the group - but you can always check what groups you belong to by clicking that link on the left.