From
http://www.spamcop.net/fom-serve/cache/306.html :
How do I whitelist yahoo groups?
Yahoo Groups mail should have a Return-Path header that looks like ...@returns.groups.yahoo.com
In order to pass all Yahoo Groups mail through to your inbox, add "returns.groups.yahoo.com" (without the quotes) to your whitelist.
If whitelisting the Return-Path of those notifications does not work for you, please post the headers from two of those messages you'd like to whitelist (so that we can help you determine which parts of the addresses are static and which parts are dynamic), along with the address that you attempted to whitelist.