This article describes why one would list their group in the Yahoo! Groups directory and how to do so. 

If you’re interested in growing your membership base, you should list your group in the Yahoo! Groups directory. It’s the only way for new members to find your group. Either public group listings can have open membership or restricted membership -- the choice is yours.

If you don't want the general public to find your group or request memberships, then choose Not Listed in your group settings.

Here’s how:

1. From the list on the left, click Management. This link only appears if you’re the owner of the group or a moderator with the appropriate privileges.
2. Under "Group Settings" in the upper right, click Description & Appearance.
3. Click Edit next to "Description".
4. Choose Listed or Not Listed next to "Listing Type".
5. Click Save Changes.