<?php
/*
Plugin Name: bPlus User per site
*/

function is_valid_user_login( $user = null ) {
    // Current site
    $current_site_id = get_current_blog_id();

    if ( is_null( $user ) ) {
        $user = wp_get_current_user();
    }

    $sites = get_blogs_of_user( $user->id );

    //var_dump($sites);
    //die;

    if ( is_user_member_of_blog( $user->id, $current_site_id ) ) {
        return true;
    }

    return false;
}

function my_check_user_blogs( $user_login, $user ) {
    if ( current_user_can( 'manage_options' ) ) {
        return false;
    }

    // check if user is in the current site
    if ( ! is_valid_user_login( $user ) ) {
        wp_logout();
        wp_redirect( get_bloginfo('url') );
        exit();
    }

}

add_action('wp_login', 'my_check_user_blogs', 10, 2);


function my_set_default_blog_to_user( $user_id ) {
    add_user_to_blog(1, $user_id, 'subscriber' );
}

add_action( 'wpmu_new_user', 'my_set_default_blog_to_user' );

function my_check_user_access_for_site() {
    // check logged in users only
    if ( ! is_user_logged_in() ) {
        return false;
    }

    // dont perform check on admins
    if ( current_user_can( 'manage_options' ) ) {
        return false;
    }

    if ( ! is_valid_user_login() ) {
        // log out
        wp_logout();
        wp_redirect( get_bloginfo('url') );
        exit();
    }
}

add_action( 'init', 'my_check_user_access_for_site' );