I still don't understand why we can have more than 5, moreover that the surplus will not train.
Well, first off, the main character is the only character that MUST be in the ground crew, and it isn't necessary that you hold ANY position on the ship crew, so it's conceivable that you could have 4 people (including yourself) trained in combat for the ground crew to perform missions, a different 5 people that run the ship, and another person whose only purpose is to get the best deals when shopping. Since that NPC wouldn't need any combat skills, you could put EVERY attribute point into charisma and get better money than an NPC that was splitting up the attributes more reasonably (to be more survivable in combat).
That adds up to 10 characters (You and 9 NPCs).
Currently, there is no reason you can't have the ground crew also manning ship positions, but it was my understanding that the developer MAY intend at some point to have things happening onboard your ship while your ground crew is actually doing things. If/when that is ever added into the game, you'll actually need the 4 ground crew to be different NPCs than the 5 ship crew, and a full roster of 10 will only allow you a single spare NPC to swap out for unusual situations (like if someone gets killed - I'm not even sure if that's possible right now, but it might be in the future)