EndNote X7 introduced a new sharing mode and EndNote X8 and X9 expand on it:
New in X9:
If you do not already have an EndNote Online (formerly called EndNote Web or Basic) account, go to EDIT>PREFERENCES>ENABLE SYNC and register for a free account to start sharing.
To sync go to TOOLS>SYNC or use the Sync Button.
Once the library is synced, go to FILE>SHARE and enter the email of users you wish to invite to share your library or use the Share Button. They will receive an email and instructions. All users must be running X7 or X8. Use HELP>CHECK FOR UPDATES to confirm you are running the latest version of EndNote.