I looked at the source of the plugin, and I think this would be complicated as the list works by associating user IDs with list IDs (which is also post ID). As unregistred users have no IDs it would be complicated. As a workaround I added the possibility to register for an other user and added anonymous user to my wordpress website, so that people can register for their unregistred friends as anonymous people. (See my thread about my patch)
Maybe that could be off interest to the two guys in thread “Attendance List clashing with All in One Event Manager” but the thread is closed, I cannot answer to them…