Nobody Saves the World

You'll find the thieves guild in the very bottom left corner of the map (shown as a tower). Once you've saved the Knights guild the gates open up and you can make your way there. As you head through the Royal Gardens and Rockrust Barrens you'll come across a guard checkpoint and get thrown into a holding cell. Inside the holding cell you'll find "The Clunk" dungeon, bare right in The Clunk till you find a member of the thieves guild imprisoned. Release him and complete the dungeon. Now again make your way to the bottom left of the map towards the highlighted tower. You'll eventually reach Tarview and the home of the thieves guild, use "Password" as the password to enter, talk to the guild leader and the achievement will pop.

After the Knights Guild opens the gates, as you travel west you will eventually enter a passage where you are arrested and tossed in a jail cell. Once you escape, there is a level 10 dungeon between your cell and the exit. Within this dungeon, you can find a key and unlock the cell of someone who thanks you and says they are from the Thieves Guild.

As you continue west towards the Ancient Robot, another member of the Thieves Guild will pick-pocket you, leaving you a note with the location of the Thieves Guild in the south-west part of the map. Steal your items back, and you'll get your Achievement for being made a member of the Thieves Guild.