I haven't seen it posted anywhere, but a long time ago, Yoshi said that those who fight level 25 Ifrit with four from the same grand company would get special rewards. Then it was kind of forgotten and I haven't seen any posts about it.
Well, last night my LS tried doing the fight with four adders. There were no drops from the fight itself, but the people who needed the quest were rewarded with 500 extra seals for the deed.
People who entered the fight (whether they needed it or not) were given a variation of the achievement in Lodestone history:
"Grover Eyeveen (Figaro) has completed the quest “It Kills with Fire” with no aid from outsiders!"
500 seals (outside from questing) are pretty hard to get, and it's also halfway to a rank promotion. So if you're coming back to the game or haven't done the level 25 Ifrit fight yet, I recommend trying to get others from the same grand company to help you.