Make sure you have a yahoo ID first (or you can get one as you sign up I think), then go to ...
Yahoo! Sports Fantasy Football
Then click "sign up now"
Then choose the "join existing league" option
Then choose "accept an invitation" option
League ID is 100965
Password is "rivalry"
and from there its just choosing team name and all that.
Hope to see you there so we can kick some cowboy tail!