This achievement is easily missable. Once you finish Main Quest: Ugly Baby, Secondary Quest: Brothers in Arms will automatically begin.
I will make sure to avoid spoilers other than the fact that these people are in the game, so you can read on safely.
The quest sends you to collect allies. Each ally you collect is tied to a quest which you must complete in order to send them to Kaer Morhen to help in the battle. You must complete the quest and convince them to go.
The achievement will unlock immediately after the final person agrees to go to Kaer Morhen. If you think you've got everyone and haven't had the achievement pop, you're missing someone.
The allies, and their quests, are below (Letho is optional, since it's possible he's dead due to events in Witcher 2):Velen
- Letho - Reardon Manor (near Lindenvale) - complete Secondary Quest: The Fall of House Reardon
- Keira Metz - complete Secondary Quest: For the Advancement of Learning, telling her to go to Kaer Morhen (as of patch 1.10, according to comments you do not need to recruit her though another says you must)Novigrad
- Vernan Roche and Ves - Temeria Partisan Hideout - complete Secondary Quest: An Eye for an Eye
- Zolton Chivay - Rosemary and Thyme - just ask!
- Triss Merigold - complete Main Quest: Now or NeverSkellige
- Hjalmar an Craite - Kaer Trolde - ask whoever is ruler
- Ermion - Gedyneith - just ask!The following will not come with you even when you ask them
- they can be safely ignored:
- Emperor Emhyr Var Emreis
- Sigismund Dijkstra
- Crach an Craite
- Cerys an Craite (If you speak to her and she is ruler, she will send Hjalmar)
- Folan and Vigi - Go with Hjalmar