In recent days, the Qatari ambassador to Afghanistan has escorted small groups of Americans into the airport, according to two people familiar with the effort who, like others, spoke on the condition of anonymity to discuss the sensitive operation. American citizens have been instructed to meet at rally points in the city, and the ambassador then accompanies them to guarantee safe passage, these people said. Qatar has served as an intermediary between the United States and the Taliban at several stages of the American withdrawal, sponsoring peace talks and serving as the first point of refuge for many evacuees.