Yahoo! Groups allows moderators to subscribe members to their groups or send invitations to join their group. If you do not want to be added / invited to groups, follow these steps:

1. Click the My Email Preferences link on the My Groups page.
2. Under Options, click on the Edit link for each email address you want to prevent from being invited/added to a group.
3. Under Group Invitations, make sure the box is not checked next to Allow group moderators to directly add me to their groups or Allow group moderators to invite me to their groups.
4. Click Save Changes when you’re done.