You must complete the game while never recruiting anyone to your party. After the prologue, Imoen is scripted to talk to you and forcefully join your party. Before she reaches you, leave the map, and you'll be in the clear. Afterward, the achievement will unlock once you complete the main campaign.
It's reported that this blocks "Friend of Volo". If you're going for both, keep a save just before you start the Thieves Guild maze so you can go back and recruit someone.