If you want to join the group, there are 2 options.
First option: go to the Yahoo group page (click here) and hit "join group" on a yahoo account. This will alert an administrator to approve of your admission.
Second option: Through the same link above, send an email to the subscribe email listed on that page, which will send a moderator a notice saying you wish to join. They can then process the request from here (a yahoo account is not needed for this option.)
Leave the group
If you wish to leave the mailing list, then you can either do 1 of 3 things.
1) Yahoo account - Load the group's yahoo page, and be signed in. Under the membership tab, hover over it and hit Edit Membership. Scroll to the bottom and there's a button to leave the group.
2) Non-Yahoo account - Load the group's yahoo page, and there's a list of emails for the group. Simply send an email to the unsubscribe address and it'll remove you.
3) Open an email broadcast. In the yahoo group signature is an unsubscribe link. Use that to unsubscribe similar to option #2.