Finally, shared site ownership is possible without having to go through the meta tag or HTML file verification process. The Google Webmaster Central Blog has mentioned that a verified user can add new users by going to the Verification Details page or the Verification home page . That (Verification Details page) page will show you information about the site as well as a list of any other verified owners.

Read more here: 
Google Enables Simpler Multiple Owner Verification for Webmaster Tools