you need to get the password from the commish, then go to:
Main Page -> Dynasty -> Hardball Dynasty -> Join a world
Once you're on the "World Select" page, there will be a list of public leagues with openings (there are none at the moment) and a list of private worlds w/ openings.
right under the list of private worlds, there's a box to enter a password to join the world you've been invited to.
Once you enter the code and punch the "join a world" button, you'll be taken into that world with a list of openings.
Find the team you want, pick a city and team nickname, then start enjoying the ride.