I want to keep a track of sharing documents in Document library. Is there any feature available in sharepoint for this?
How can I Do this?
You can leverage the new Security events introduced in SharePoint 2013. On every GroupUserAdded, GroupUserDeleted, RoleAssignmentAdded, RoleAssignmentDeleted event you can add an entry to a list or database that will help you track the sharing of document.
Security events available to leverage in SharePoint 2013 are given here.
A tutorial on how to do this is given here.
You can enable auditing for each site collection for which you want to collect data. Instructions for doing so are available at https://support.office.com/en-gb/article/Configure-audit-settings-for-a-site-collection-a9920c97-38c0-44f2-8bcb-4cf1e2ae22d2
This provides data on (depending upon what you enable):