Using EndNote X8 (and later versions), you can share your entire library - including references and attachments - with up to 100 other users (400 with EndNote 20.2. To do this, click File > Share.
Enter the email addresses of users you'd like to share your library with, separated by commas, and then click Invite to send out the invitation. Once accepted, the invited users will be able to add and edit references, annotate PDFs and more all at the same time.
You can share just one library on your computer with others, but there is no limit to the number of libraries which can be shared with you.
For more information, see EndNote's support page for library sharing.
