Originally Posted by daenius
Hm, that's a hard one.
I tried boosting it with somebody a couple of months ago - sprint race, created a party in multiplayer mode, it was just the two of us - and it didn't unlock until there was an "outside player" in the session.
I can't really say that it's not boostable with friends only, though, maybe it just needs a certain amount of players to unlock (in our case it was 3 or 4 players).
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.