Originally Posted by Masterbelg
I'm not certain about being able to boost the ranked gp win,
But you definitely have to chose grand prix and not sprint race.
And I got it with 2 people (me and the other guy) in the lobby, so you can unlock it with 2 people, maybe just not with only people of your party. You'll need someone else to confirm that.
I'm absolutely sure that I got this Achievement in a sprint race.
It was one of the first ones I unlocked and I only did sprint races for the first month or so!