Horizon Life is the connected world where you see human opponents on the roads.
While in Horizon Life drive to any event location and set up a race. You should be able to choose between Solo and Co-Op. Select Co-Op and wait for other players to join you. If a player joins complete the race, do 5 for the Achievement.
Note: You need to have unlocked this (and completed the race suggested to you when you do) first in order to access Multiplayer: