I've checked with Brighttalk Support and this is what they have responded with:

From what your viewers described, they are visiting your site and clicking to attend the webinar.

If they are not logged into their BrightTALK account on their browser, they will be presented with a registration form (for joining BrightTALK). There should be a button on the top right of this form to switch it to a Log In form.

It is possible that they either did not notice this, or that their browser version or settings prevented this button from appearing on their screen.

Support is happy to handle your inquiry/issue directly. If you'd like, please forward your e-mail address to me at tgaron@isc2.org and I'll connect you with with my support contact.