To join the Thieves Guild in The Elder Scrolls IV: Oblivion, follow these steps:

• Go to the Imperial City Waterfront District, specifically to the Garden of Dareloth, which is behind the Abandoned Shack south of the pirate ship.
• Arrive there at midnight, where you will find a gathering of thieves. Talk to Armand Christophe when he arrives to begin the initiation quest.
• The initiation quest involves stealing a book from Amantius Allectus' house in the Temple District. The house is located by entering the Temple District from the Waterfront, then turning right and following the path along the wall until you find a door on your left. Inside, find the desk on the right wall and take the book.
• After retrieving the book, return to Armand to officially join the Thieves Guild.

Alternatively, you can trigger the guild invitation by committing a crime and serving jail time, after which you may be approached with an invitation. Another method is to raise the disposition of a beggar in the Waterfront District and ask about the Gray Fox, who is connected to the guild; the beggar will then direct you to the Garden of Dareloth

. Joining the guild allows you to sell stolen goods through fences and pay off bounties for less than the normal cost, making it beneficial for characters skilled in stealth and security