You add new attendees to a session through the "Add/remove attendees"-page. You select multiple users from the list. You try to add them with the arrow button but run into some error with the confirmation email (the mail server isn't properly configured or no email-address has been defined for the user). If the confirmation email can't be sent, the users will not be added to the session at all. Also, after the error the user is returned to the moodle frontpage, and not back to the "Add/remove attendees"-page. This can be somewhat confusing, the user doesn't get any feedback whether the students were added to the session or not.
Timo