Join via the group’s page:

1. Once you’ve located a group to join (at groups.yahoo.com, you can browse the Groups directory or search for a group by topic), click Join This Group in the upper-right corner of the group’s page.
2. Haven’t signed in yet? You’ll be asked to enter your Yahoo! ID. If you don’t have one, register by clicking Sign Up at the top of the page.
3. Set your membership preferences. Choose a profile you'd like to display to the group, select the email address at which you'd like to receive group messages, choose how you receive group messages, and more.

Join the mailing list only:

Yahoo! Groups recommends that you join a group via the group’s page (above). But if you want, you can simply sign up for the mailing list for the group. When you’re only on the mailing list, you don’t get access to all the group's web features (Photos, Files, Links, Polls, Calendar.)

To subscribe to a group's mailing list via email, send a blank email to:
groupname-subscribe@yahoogroups.com

Make sure to replace "groupname" with the actual name of the group (e.g., pastry_chefs-subscribe@yahoogroups.com).

When you get the confirmation message, just reply to it…and your subscription will be complete!

Respond to an invitation to join:

If someone sent you an email invitation to join a group (and you’d like to join), click Join this group!