Why aren't all of my groups listed on the My Groups page?
This article explains why some of your Yahoo! Groups may not appear on your "My Groups" page and how to make them appear.
When you sign into Yahoo! Groups, you use a Yahoo! ID. This ID is used to recognize you whenever you access online features of Yahoo! Groups.
When you join a group, it is your email address that’s subscribed, not your Yahoo! ID. In order for us to know which groups you’re a member of, you must use the membership wizard to link your email addresses to your Yahoo! ID. Once that’s completed, you’ll see all the groups your email address is subscribed to listed on the My Groups page.