I take this from: http://guidesarchive.ign.com/guides/13437/optional.html
"Acquiring a Thieves Guild
In the Docks area talk to Renal Bloodscalp in the Shadow Thief Guild (the apparent one - not the hidden one). He'll ask you to dig up some dirt on a fellow guildmaster, Mae'Var. Head to Gorch's place and show him the papers that you got from Renal. He'll let you in to the guild. Wander around and loot the place - try all those door traps on the second floor for a lot of experience! Then head to the bottom floor and talk to the guildmaster. He asks you to complete some tasks in order to earn his trust.
He asks you to retrieve an amulet from the temple of Talos in the Temple district. Just head there and enter the place. Go to the bottom room and you'll see the Weathermistress sleeping on the bed. Pick the lock on the chest and swipe the amulet. Bring it to Mae'Var.
Now he'll tell you to talk to Edwin on the second floor. Edwin requests that you kill a Cowled Wizard that has been threatening him. Good. I always wanted to slay one of them! Head to the open building near the Harper Hideout and kill the mephits on the first floor. There are two Stone Golems on the second floor. The Cowled Wizard is on the third floor. He is easy to take on if you have the Breach spell. If you don't, get ready to cast some holding spells to keep him still, and dispel magic to get rid of his weaker defenses. Now return to Edwin.
Edwin asks you then to retrieve some papers from Marcus in the tavern a few doors down. Threaten the guy to get his papers. Return to Edwin with them. Now head back to Mae'Var to get his last task. Return to the tavern and go to the second floor. Talk to Embarl and get his dagger from him peacefully (some characters might revolt if you kill the guy!). Give it to Mae'Var. Head upstairs and talk to Edwin again. Tell him the truth and he'll give you the key to Mae'Var's chest and will join your party. Use the key on the second floor chest to get the documents and a pair of Boots of Stealth. Bring the documents to Renal Bloodscalp.
Renal gives you his authorization to kill Mae'Var. This is important, if you are a thief! This is a stronghold quest! This is simple, really simple. Just return back to Mae'Var's guildhall and attack the thieves in the entrance. They are not tough at all. Then head downstairs and kill Mae'Var. Not too hard. Return to Renal for your reward! (He'll offer the guild to you, but only if you are a thief!) "
I think that you can't get Edwin in your party now. I think that you can find edwin in the last floor of the thives guild, but i dont remember, since I finished that game a lot time ago!!