when the CM link is browsed users should be automatically logged in to the CMS.How do i do this ?which pipeline should i over ride .
Sitecore's AD Module provides functionality to allow users to perform Single Sign-on to the Sitecore client using their Active Directory accounts. As the users land at URL /sitecore/admin/ldaplogin.aspx it will authenticate them.
You can read more about how to configure LDAP login as the default login at http://www.nehemiahj.com/2013/01/how-to-enable-single-sign-on-in-sitecore.html
Alternatively you can set your CM domain default page as /sitecore/admin/ldaplogin.aspx.