Endnote Web a really useful tool for collaborative work as it has the ability to share groups of citations with other people . As long as both you and the person with which you want to share a group of citations have Endnote Web accounts, sharing citations is very easy. To register for Endnote Web see this post.
The instructions for sharing groups of citations are below.
1) Click on the Organize tab
2) Click the Manage Sharing button next to the group you want to share
3) Click the Start sharing this group link
4) A new window will open. Enter the email address of the person(s) you want to share the group of citations with (Note: the email address you enter needs to be the same one that the person you are going to share with has associated with his/her own Endnote Web account). Select either Read only or Read & Write privileges. Click the Applybutton and then the Close window link
5) To review your sharing settings. Click on the Organize tab at the top of the page. Check the box in the Share column next to the group you want to share
Note: Once you have shared a group you will see an icon next to groups you have shared on the My References page indicating it is a group you are sharing with others