EndNote includes a feature that allows you to compress and email selected groups. So, you would first want to create groups for each author in your EndNote library, and then use this feature to send them their specific group of references. See the attached screen shot with the relevant options highlighted in red. They could then send you an .enlx file back that you could incorporate into your library. Alternately, you could consider sharing these groups via EndNote Web and then you would not need to send files back and forth via email.