You need to speak to their recruiter in the marketplace during the day, where you'll be given a small series of tasks to complete before the guildies in the sewers start to deal with you.
To join the Thieves Guild the Dragonborn must enter Riften during the
day, and speak to Brynjolf, a red-headed Nord male, who can be found
in the Riften Marketplace between 8:00 a.m. - 8:00 p.m. Once the
Dragonborn is within proximity, Brynjolf will automatically attempt to
strike up a conversation in which the Thieves Guild prerequisite quest
A Chance Arrangement can be started. Alternatively, in the evenings
past 8:00 p.m., the Dragonborn can encounter Brynjolf at The Bee and
Barb inn, at which time he will tell the player to speak with him
during the day at his stall in the Riften Marketplace.
From: The Elder Scrolls Wiki