With WebinarJam, you can invite both paid and unpaid attendees to a webinar. Set up the webinar as a paid event, and the registration page will require payment. Then, send the “Generic Webinar Live Link” to an attendee for whom you wish to waive the registration fee, and they will be allowed to join the session without being required to pay.
The downside? The analytics are based on the individual links specifically created for each person that pays the registration fee. Anybody who logs in as an attendee with a generic post-register link will not be tracked, which skews your statistics.