The way i did this is my friend just sent me a inv to watch replays, so all u have to do is to at least watch 1-2 replays, with ur friend/friends in the Watch With Friends in Replay mode online and look at some together=) Very easy achievement!!!
P.S. make sure u have friends with the game=p
EDIT: Apparently some people didnt unlock this if they wasnt hosting the replay room, So if this is the case then whoever is hosting will ONLY get the achievement. So just take turns hosting the room after you get the achievement so that your friend/friends can get the achievement too!!!