some reasons why people join.....
http://faq.distributed.net/cache/29.html
you need to download the client from:
http://www.distributed.net/download/clients.php
Unzip to a folder and start the program (dnetc.exe). Configure the email address to be used (under General Client Options) and start running the client. It'll fetch blocks from the keyservers and start the crunching. More details here:
http://www.distributed.net/docs/tutor_clients.php
To join a team, you need to get a password first. Do a search for your email here
http://stats.distributed.net/projects.php?project_id=8
Near the bottom you could request for the password to be emailed.
With the password, you could then click on the following link and click on "I want to join this team!" near the bottom:
http://stats.distributed.net/team/tmsummary.php?project_id=8&team=457
There may be shorter way though ;p I'm a newbie... just joined a few days back.