Can't edit the citation added by other users from a shared group and the bibliograph can't be merged

I'm editing a document from my college. We have a shared Mendeley group. However, Mendeley always generated a new separate bibliography when I add any new citation, other than list the new citation at the bottom of the original bibliography. In addition, I can't edit all the citations he added before. Does anyone have an idea how to solve this issue? Thank you very much! Ninggang