One of the most sought after features in Google Webmaster Tools was to easily give third-party people access to your Google Webmaster Tools account without giving over your own username and password and without giving them full control of your site.

Google has finally satisfied this request by adding user administration to Google Webmaster Tools.

Now, site owners, who are defined as an owner if they go through the verification process in Google Webmaster Tools, can add access for others without making them go through the verification process. The access granted is either “owner,” “full” or “restricted” access.

Owner Access – Anyone who has verified the site in Webmaster Tools. This enabled them to add users via the user administration, delete sites, and more super controls.
Full Access – Granting a user “Full” permission means that they will be able to view all data and take most actions, such as changing site settings or demoting sitelinks.
Restricted Access – When a user’s permission is set to “Restricted” they will only have access to view most data, and can take some actions such as using Fetch as Googlebot and configuring message forwarding for their account.

You can add users either on the overview page where it says “manage site” and then by clicking on “Add or remove users” or under the “site configuration” section where it says “user administration.”

Sources:
http://searchengineland.com/google-webmaster-tools-finally-adds-user-administration-113865