Explanation/Reference:
Note:
Add users to a permission policy
You might want to add users to a permission policy to ensure that all users are accessing content with the same set of permissions.
In the Policy group of the ribbon, click User Policy.

In the Policy for Web Application dialog box, select the check box next to the user or group that you

want to manage, and then click Add Users.
In the Add Users dialog box, in the Zone list, click the zone to which you want the permission policy to

apply and then click Next.
In the Add Users dialog box, in theChoose Users section, type the user names, group names, or e-

mail addresses that you want to add to the permission policy.
In the Choose Permissions section, select the permissions that you want the users to have.

In the Choose System Settings section, check Account operates as System if you want to specify

whether a user account should be displayed as SHAREPOINT\System instead of the actual accounts that perform specific tasks within the SharePoint environment.
Click Finish.

Reference:https://technet.microsoft.com/en-us/library/ff608071.aspx