I'd check with WebEx on that one. There probably is, I believe if you check in the webinar creation box to find that link.
You could also re-run their registration or ask them to check their calendar.
Also, to my knowledge, WebEx doesn't allow more than 1k attendees to a webinar. So, while you can have over 1k registrants, not everyone will be able to join. In your reminder email that you send out before the event, you may want to mention that "space is limited" if you don't already plan on doing that. Good luck!
You can use the send email action on the person record. Just find the person in lead database (not in program membership view, though as it does not allow you to make marketing actions), then select action Marketing/Send Email. If the email uses the member URL token, it will be populated.
If the person is having issue getting emails from Marketo (because Marketo is blacklisted by their company email server), then you can use the send alert function to send the confirmation email including the personalized link to your mailbox and then forward it to the registrant, which would avoid the Marketo blacklisting hurdle.