Go to "GRID Online", then "Events", then "Xbox LIVE" and select "Online Playlist". Now pick any of the 3 playlists available which include "Everything", "Racing" & "Alternative". Once you join one of these playlists, you cannot leave the lobby for at least 5 events. You do not have to win or finish for the races to count. A DNF or Disqualification will still count as taking part in the race. Just keep playing until the achievement unlocks. Picking the Racing playlist, this shouldn't take more than 20-25 minutes.