Refers to the group of people working on a ship, aircraft, or spacecraft. Often used in compounds like 乗組員 (crew member).
All of the ship's crew were safe.
The crew of that airplane is ten people.
A 乗組 (crew) is counted as a 組, i.e. as one group or team of people working together.
crew member; crew
Compound of 乗り (nori, 'riding') + 組み (kumi, 'group'). The kanji 乗 means 'ride' and 組 means 'group' or 'team'.