You must get a full game going for this (4 people including yourself).
All 4 ships must attach together, it will pop as soon as you 4 are connected so there is no need to stay together if its not wanted.
To attach to another ship simply get close to it and press
You can usually announce you need the achievement and everyone will come together or you can get 3 friends to help you out.
Also if you happen to have 3 extra controllers you can do this yourself locally.