Navigate to the live event you want to make free under Content > Live Events. Under Pricing select Free for Everyone.
If you want to know who gained access to the live event, there is a small workaround for this: set the Live Event as "Require Access" (instead of "Free for Everyone" on the Pricing tab) and add the Live Event to a Freebie Bundle. This way, when a user clicks on the Live Event to Get Access they will instead be able to claim the freebie bundle by signing up, if they don't already have an account, without having to pay or subscribe to your service.
If you want users to participate in the live chat, encourage your users to sign in (or use the freebie bundle approach which requires access to view the event despite it being free) so that they can engage with you and others watching the event.