A few days after making your request to join yahoogroups you should receive an email which looks like this. Click on the "Join this group" button.

A browser window will open, and the Yahoo Welcome page will be show. Click on Join the Group and click CONTINUE.

You can either sign in with your existing Yahoo account, or register for a free account.

Follw the instructions to register for you new account, if you don't already have one.