Hypothesis (completely unproven): The top X people on agro participating in the FATE receive gold rating. Being in a group somehow let's you take credit for your group's agro.

So if 1 person solo's 99% of the FATE and 5 people show up just at the end and tag it with smallest ability, then all 6 should get gold assuming X is at least 6.

Anecdotal evidence: I've showed up late and done almost nothing to get gold. And in a high participation FATE warrior just run up, hit 2x Butcher's Block (high agro, low damage) then leave and still get gold.