To my colleagues in the Program for Writing and Rhetoric: this screencast demonstrates the method I recommend using to add students as members to your class blog on Wordpress.com

Students only need to become members if either (a) you want them to post new entries on the blog, or (b) the blog is visible only to logged-in members. If you will only ask students to post comments on blog entries, they don't need to become members of your blog. In fact, they don't even need to have accounts on wordpress.com in order to post comments, depending on the settings you chose under Discussion (under Settings) on your blog Dashboard. Contact me for more details!