There are 7 allies that you must recruit for the Full Crew. After doing the main quests in Kaer Morhen (towards the end of the story) you will receive a main mission called "The Isle of Mists" and 4 other "Brothers in Arms" quests. That's when you are supposed to recruit people for your upcoming battle in Kaer Morhen. You must play a series of lengthy side quests and subplots to recruit everyone, which is going to take several hours. Keira Metz and Triss can be recruited before reaching this point in the game and are the easiest to miss.