When you create a reunion for your class, you'll have the option to invite additional classes from your school. On the Create a Reunion page, simply select additional classes under the Invite by year menu. All classes you've invited will receive an email invitation and any updates you post.
Note: There's no option to invite more classes to a reunion after it has been created. In that situation, you may want to create a new reunion and specify that the new one was created in order to extend the invitation to additional classes. Both reunions (the original and new one) will be posted on the site.