Adding capability to users

Hello World 🙂

I’m trying to let some normal users (with Author Role) to moderate all the blog comments (and not only theirs)

I read http://codex.wordpress.org/Roles_and_Capabilities and I thought that I just have to do something like :

$user_object = new WP_User($userID);
$user_object->add_cap('moderate_comments');

I check the usermeta table in database and I find that effectively the given user has a new metakey blogtest_capabilities :
a:2:{s:6:"author";b:1;s:17:"moderate_comments";b:1;}

But when i log with my given user, it seems to not work and the user still can’t moderate other comments ?

Have I miss something ? May be I have to create a new Role that extend Author role and add the moderate_comments capability ?

Best regards