Gamer has been removed
EDIT 20-feb-2014
I am really sorry but Rockstar has been patching everything. I suggest boosting this achievement through a session on trueachievements. My method was just a little easier to do and required less hassle
This means step 5 and 6 don't work anymore and have been replaced
My original method:
It's even easier to get with 1 friend. --> You need 4 people incl yourself
1) Set any two seater-landvehicle (so bicycles excluded) race to Rally type.
2) Have the others prefer the driver role. And prefer yourself to be the co-driver.
3) start the race without anyone else joining (by setting the lobby system to closed)
4)You can direct the driver by using left and right on the d-pad. The driver cannot see the checkpoints (hence the purpose of the rally race)
5) Have the driver quit the session, by switching to a single player character with down on the d-pad. Making you be the only person left on the "Job"
6) Achievement will unlock when it says "WINNER"
5) Win the race and the co-driver will unlock the achievement
Extra info: even if you prefer certain roles it might not always assign you to them. So this might take a few tries
If you give bad feedback, post in the comments why please!