Not long after arriving in the City of Venice's Outer City, Scarlett will need to choose to join one of the city's three guilds. (She'll need to show a guild badge to get Nox to trust her, and without Nox to provide information and teach her some necromantic Mental Skills, Scarlett won't be able to complete the Nightwatch Troubles Quest and get the key to the Inner City.)
The choice of Guild will determine a few optional Quests which Scarlett can complete. In order to join a Guild, Scarlett will have to complete a Quest as a test. After that, each guild offers additional optional quests.
Two of the guilds offer training, but they will provide training for members of other guilds as well, so the choice of Guild won't restrict Scarlett's choice of Skills.
Each guild also offers Scarlett a bed in their guildhall, should she wish to rest, recover energy, or just blow off some time on the clock.
The Order of the Holy Seal is an order of warriors, motivated by anger, rage and a thirst for revenge. Scarlett can discuss joining with Rangar at the quildhall's courtyard in the northern part of the Outer City. (Training in Physical Skills (combat) is available to member and non-member alike in their courtyard from Rangar , in the Outer City.) If you are interested in the evil ending, this is the guild for you.
The Net of the Mask is a secret order of scholars and necromancers, who publicly deny pursuing the forbidden dark arts. They espouse a hunt for knowledge (though they sound kind of angry, too.)
Although the Net of the Mask Headquarters is in the Inner City, Scarlett can discuss joining with Aeris on the Outer City courtyard by the gate to the Inner City. (Once inside the Inner City, Aeris will offer training in Mental Skills at the Mask's headquarters regardless of which Guild she chooses to join.) This guild is considered good, despite being necromancers.
The Hooded Wings is a Guild of messengers, who transport various packages from one part of the city to another, no questions asked. They value obeying orders, regardless of what's being delivered to whom.
In terms of Venetica's story, the Net of the Mask guild is probably the most interesting to join. Victor was once a member of the Net of the Mask, and the quests assigned by guildmistress Aeris will all have to do with unearthing Victor's history before he became the Undead Archon.