Question: How does Crew XP work if I am in more than one Crew?  Do all the Crews I'm in get XP or just the one that is active when I am playing that Multiplayer game?

Answer:  Crew XP applies to the Crew that you currently have selected as your active Crew in multiplayer. In other words, if you are wearing that Crew's emblem, that's where your XP earned will go.