Yes, all attendees must register first before they can attend a webinar. The minimum amount of registration information a webinar Organizer can require is the attendee's name and email address.
It is not possible to disable the webinar registration, though if you're working with a developer or third party integrations specialist (such as Zapier), then you may be able to import an existing client list to each webinar you set up.
Hi Lee, that is from our developer site for those that use our free APIs to integrate with GoToWebinar. Until we are confident that we have measures in place that will prevent abuse of a feature like this, we will not enable bulk uploading of registrants. Without the right checks in place, an Organizer could register anyone without their knowledge and then use our system to spam them.
There are many providers that can also assist you with importing contact lists to your webinar registration pages if needed: https://www.gotomeeting.com/integration-marketplace