The crew is a singular collective concept, it is all of the people that work on the ship. The crew is singular but it means a group of people. There is one crew, but more than one member of the crew. I hope that helps.
In this case crew is a noun, but it's still singular it's like the word crowd, there is one crowd, but there are many people in the crowd, there is one crew, but there is more than one member of the crew