Welcome to our forum, @Ying_Chen! You are correct – at the moment items cannot be cited directly from shared folders, so you would have to save the shared papers to your library in order to cite them.
At the moment we are working on revamping our sharing structure to improve this workflow and offer more collaborative features. Check out our roadmap for that and other plans – significant changes for our extension and interface are currently in beta; new sharing features should be rolled out within the next couple months.