When comparing achievements with a friend, I saw this one under the feats of strength tab. What is this? When will we be able to get it and how do we get it ?
According to this post on the official forum; it appears that the achievement can be unlocked by playing a game with a guest player in your party until completion (no rage-quitting or early leaving or so).
The easiest way to achieve this would be to give one of your Starcraft 2 Guest Passes to a friend (or use it yourself to play the game using 2 different PCs).
The achievement is apparently usually awarded at the Experience or Score screen.
The achievement is only rewarded by inviting a Wings of Liberty player into your HotS party and completing a game. the Achievement is rewarded only if both players finish the entire game and you will both receive the Achievement at the score screen.
You can do this for any WoL player, and anyone who already has the achievement will still be able to help you get yours.