The Assassin Achievement
Successfully recruit the Assassin
These achievements are story related. You will be given two dossiers, which contain all the locations of each crewmember.
Meeting the Illusive Man Part 1:
After your first meeting with the Illusive Man, he will give you dossiers on recruitable squad members. The game recommends you go to Omega first for Mordin Solus and Garrus, but you can do them in whatever order you wish. When you reach the planet with a squad member and land, you will get a mission. You will recruit the squad member upon its completion and the achievement will unlock.
Note: The Krogan achievement will not unlock at the end of the mission like the others. It will unlock when you go into the cargo hold and free Grunt from the tank.
Meeting the Illusive Man Part 2
After meeting the Illusive Man for the second time (after completing Horizon), you will receive the dossiers for the last group of recruitable squad members. You can recruit them in any order you wish.