Dave Harrison, Jr., Mary Gilbert, and Cynthia Daniels (plaintiffs) were in a vehicle collision with a tractor. Harrison was driving the vehicle in which Gilbert and Daniels rode. The plaintiffs sued M.S. Carriers, Inc., which owned the tractor, and others (defendants) in a Louisiana state court, for injuries allegedly sustained in the accident. The defendants removed the case to federal court on the basis of diversity. Gilbert and Daniels then sought to amend their complaint to state negligence claims against Harrison as a defendant, as well as his insurance company. Harrison did not have diverse citizenship from Gilbert and Daniels. A magistrate judge denied the motion to amend without prejudice, asserting that Gilbert and Daniels could bring a cross-claim against Harrison and an amended complaint against his insurer. Gilbert and Daniels sought review of the magistrate’s order.