Here are a few ways to do that:

1. Click Edit Membership at the top of the group’s home page.
2. In Step 3 at the bottom of the page, select "Fully Featured".
3. Click Save Changes.

Within an email from the group, you can also click change delivery settings in the email footer.

Finally, you can make this change via email by sending an email to "GROUPNAME-fullfeatured@yahoogroups.com" where "GROUPNAME" is the name of your group.