Allow Administrator role access to custom capabilities [duplicate]

Referring to the Codex entry for this, I suggest using another hook. They use the ‘admin_init’ action:

function add_theme_caps() {
    $role = get_role( 'author' ); // gets the author role
     $role->add_cap( 'edit_others_posts' ); // would allow the author to edit others' posts for current theme only
}
add_action( 'admin_init', 'add_theme_caps');