Brothers in Arms: Novigrad
This quest begins automatically upon completing The Ugly Baby Story Quest. For this quest you'll need to travel to Novigrad and speak with three different people to ask them if they'll join your fight at Kaer Morhen. If this is the first Brothers In Arms quest you're doing then let me tell you how it works; in order to ask the person for help you will typically need to complete their quest chain.
In Novigrad each of the people you have to ask is tied to a quest chain that involved them earlier in the game. Typically these involve side quests but for some people (like Sigismund Dijkstra) it involves a story quest. To make things easier, below is a list of all the people you can ask for help and the required quests.
Zoltan: Complete A Dangerous Game and he will join you.
Roche: Complete the quest An Eye for an Eye and he'll join you. Ves will join your team as well.
Sigismund Dijkstra: During the quest Count Reuven's Treasure if you asked Mende about Dijkstra's treasure then he will agree to help you.
Triss: Complete the quests A Matter of Life and Death as well as Now or Never and Triss will agree to help. Note: If you've completed these quests this update to your quest is automatic, Triss will be waiting for you at Kaer Morhen.