Unfortunately, you do need to create an insanejournal account in order to become a member. However, once you do that you'll be able to join other communities and enjoy their locked fic/art as well. I'd say it's well worth the few minutes it'll take to create a profile.
Go to insanejournal dot com and create a profile. Then come back here, click on user info and join the group. All the NC-17 goodies should be unlocked for you then.