This isn’t a direct capability of Group Funnels, but there is a way to attribute referrals to certain people as they get sent to your group. You’d have to have a few things in place:
An affiliate tracking system (like Clickfunnel’s Backpack software)
Registered affiliates with their own links to send traffic
A landing page with a link to your group
That way, anyone who sends members to your group can be credited in your affiliate tracking system in case they become customers!