Originally Posted by Cheeese
Can anyone confirm that you can boost the ranked grand prix win?
because it doesnt say whether you can or cant in the guide,
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).