What is your webinar platform?
We used WebEx at my previous company, and I used to send the ungated access link through my work email to all internal employees. I also would create a calendar invite for them with the internal link so they could easy access the day of and not need to register. We now use ON24, which has a similar access link. I wouldn't recommend sending an email to internal through Marketo, unless you're ready to pull employees out of your registrations and attendees using Smart Lists.