When the 'share session' link is created you will notice that it has a URL in it with an IP, Server Name, or full qualifed server name in it.
It is that reference which the end user MUST have access to join a collaboration. It makes sense that your users can do this
If you have a QlikView deployment on the extranet AND you access the app through the extranet the URL will be generated with the internet accessible server reference which your users may have access to
http://<Extranet Server reference>/....
In essence, there isn't anything you have to do to make session sharing work in an extranet environment except the one big thing of reconfiguring your QlikView Server with ports, firewalls , web servers, proxies etc... to be internet accessible.
Consider security configuration and licensing implications (for normal users not session collaboration joiners) as you do this as well.