There is three ways to do this, boosting with a friend is one, the other two I will explain below.
The first method you can try is by going to multiplayer and then going to custom game, it will search to see if anyone is hosting a championship. If they are simply join up and complete the race. Simply joining will not give the achievement you have to finish the entire race.
The second method is to create one yourself, when naming it you can use something catching like Achievement or More to Come Boosting. Be sure to go into race options and turn off qualifying and set the laps to three, a short race is more enticing for people to join. You only need yourself and one other person to start the race.
The online is pretty dead in this game so if anyone needs help message me on Xbox.