There are two ways in which you can subscribe to a Yahoo Group.
1) By using the yahoo id.
 It's simple. Just go to the group which you want to join and click on Join this Group. If you are not logged in you will be asked to login to proceed further.

2) Without using the yahoo id.
There are two ways you can join the Yahoo Group without using a Yahoo Id.
Open a yahoo group. There will be a email for subscription.
GROUPNAME-subscribe@yahoogroups.com
Just send a mail at this email address. You will get an auto response for verification. Once you click on verification link you will be asked to join the group.


Secondly, you can go to http://groups.yahoo.com/subscribe/GROUPNAME to subscribe.

When you join a group without using Yahoo Id, you will not be able to access some features.