spn_j2_xmas is currently holding sign-ups on LJ, here. If you're interested in playing, please check the post out - a run-down of rules is available there, and sign-ups remain open until November 17.
For IJ-based people without LJ accounts - we will find a way to accomodate you. You can sign up using OpenID, and when the time for posting comes one of us mods will post to the community for you with the link to wherever you have hosted your creation. Email us at spnj2xmas@gmail.com to discuss specifics! If the interest from here is big enough we will set up a feed as well, so you can comfortably follow the proceedings.