To join an EndNote library that has been shared with you, you must first accept the invitation in your email. From here, you must create an EndNote account and connect it to your desktop. To do this, open EndNote and select Edit > Preferences > Sync. Select "Enable Sync" and follow the instructions to set up your account.
Once the invitation is accepted and your account is connected, select File > Open Shared Library to view and edit the shared library. Any edits or modifications you make to the library will be shared with all connected users, and all users can edit the library simultaneously.
For more information about sharing libraries, see EndNote's support page: http://endnote.com/product-details/library-sharing
