If you received a notification asking you to "complete your account," this means that the email address you've used to join the group is not currently connected to a Yahoo! ID.

When you receive this email notification, click on the link provided, and you’ll be taken through a very simple and brief process of linking your email address to a Yahoo! ID. If you do not have a Yahoo! ID, you’ll be given the option to create one.

By completing your account, you’ll have access to all of your group's web features (previous messages, photos, files, calendar, etc.) and easier control of your message delivery options. It's easy, fast and free!