login-redirect.php

<?php
/*
Plugin Name: Simple Login Redirect
Plugin URI: https://premium.wpmudev.org/support/
Description: A simple mu-plugin that redirects users to their subsite on login.
Version: 0.0.1
Author: James Morris - WPMU DEV
Author URI: https://premium.wpmudev.org/profile/jrmorris77
Network: true
*/
function my_login_redirect( $redirect_to, $request, $current_user ) {
	$current_user = wp_get_current_user();

	if ( $current_user ) {
		$user_id = $current_user->ID;
	}

	if ( isset( $user_id ) ) {

		// Get the blogs of this user
		$user_blogs = get_blogs_of_user( $user_id );

		// Get the URL of the blog
		foreach ( $user_blogs AS $user_blog ) {

			if ( !isset( $userblogid ) ) {
				$userblogid = $user_blog->userblog_id;
				return $user_blog->domain;
			}

		}

	// If user has no blog, return to the homepage.
	} else {

		return home_url();

	}
}
add_filter( "login_redirect", "my_login_redirect", 10, 3 );
?>