Mendeley is actually a suite of services that may be particularly helpful to graduate students.  By creating an account you can:

• Generate citations and bibliographies.
• Manage and annotate PDFs.
• Collaborate with a select group of researchers, or 

 Mendeley is owned by the for-profit academic publisher Elsevier, which is somewhat notorious for sending article takedown notices to competing platforms like Academia.edu and institutional repositories like the University of California system.  Mendeley is particularly notable for its ability to allow you to collaborate with a small group or network more broadly.  

If you're simply looking for a citation management tool that allows you to collaborate with a specific group, I recommend Zotero, from the non-profit Center for History and New Media.