Want to be able to redirect readers to another wordpress site but still be able to access the back end of current site

You can do it with the “template_redirect” hook. For multisite or simple WordPress. e.g:

add_action('template_redirect', function(){

    $user = wp_get_current_user();
    $userRoles = $user->roles;

    $url = site_url();

    if(!in_array('administrator', $userRoles) && !is_front_page()){
        wp_redirect($url);
        exit;
    }

});

EDIT
is_front_page() will work only if “Front page displays” is set. Use is_home() if not.

EDIT #2
Accordind with our discuss below, an example for when you play with multisite and you want redirect according to the current site:

add_action('template_redirect', function(){

    $user = wp_get_current_user();
    $userRoles = $user->roles;

    $url = get_home_url(3);

    if(!in_array('administrator', $userRoles) && get_current_blog_id() != 3){
        wp_redirect($url);
        exit;
    }

});