Powershell Script:
# --------Input Parameters
$groupName="NT AUTHORITY\authenticated users"
$siteURL="http://servername:1111/"
$listName="Shared Documents"
$site=Get-SPSite $ siteURL
$web=$site.RootWeb
$list=$web.lists[$listName]
$collRoleDefinitions = $web.RoleDefinitions;
$collRoleAssignments = $list.RoleAssignments;
$list.BreakRoleInheritance($false);
$user=$web.EnsureUser($userName);
$roleAssignment = new-object Microsoft.SharePoint.SPRoleAssignment($user.LoginName, $user.Email, $user.Name, $user.Notes);
$collRoleDefinitionBindings = $roleAssignment.RoleDefinitionBindings;
$collRoleDefinitionBindings.Add($collRoleDefinitions["Contribute"]);
$collRoleAssignments.Add($roleAssignment);
$list.Update();