Upstairs, on the balcony overlooking the guildhall entrance, Scarlett should speak to Master Deimos, who is apparently head of the guild. After asking him about working for the Hooded Wings, she can ask to join the guild. Master Deimos will then ask Scarlett to perform an initiation service for the guild.
This quest is mandatory if Scarlett chooses to join the Hooded Wings guild, and is not available if she joins a different guild first.
Return back down the stairs to the main hall, and take the side door to the southwest (to the left coming down the balcony stairs, to the right when entering the exterior door). This will take Scarlett to the Outer City Catacombs where she will find Grimm standing in the darkness to the right.
Earn the Wings - Scarlett must deliver a letter to someone in the rooftops of the Outer City to prove her abilities. This is a subordinate quest that must be completed as part of "The Hooded Wings" quest.