Crew members are characters you recruit and assign to your ship. Each crew member has a specialty — trading, combat, navigation, engineering — and a set of skills that improve as they gain experience. The right crew for your playstyle can dramatically improve your effectiveness in their area of focus.
This is a feature original to Sectorum — TradeWars 2002 didn't have it. It adds an RPG layer on top of the classic space strategy formula, giving you a character progression system that's entirely separate from your ship's upgrades.