Hi Andrea,
Through GoToWebinar, the registration is always required. At the very least the first name, last name and email address need to be entered in order to join a session. If the sessions were done under a GoToMeeting plan there is no registration, but you would have less features for moderating a large group as GoToMeeting is meant to be more of an open forum. Also, the max number of attendees that you can get through the GoToMeeting plans is 100, where as GoToWebinar offers plans for up to 1000.