One of the main new features will be available in the new wordpress version is the merging with WordPress Multi user platform, it will be called WordPress Multi Sites or Network. here are some steps you need to take in order to enable the network feature.
1. first you need to add this line to your wp-config.php file:
after that you will see a new option under Tools, it will be called Network.
Now you need to follow the regular WordPress Mu steps and then press the install button.
2. after installation complete you still need to make some changes, add a new folder in the wp-content folder called blogs.dir, it is a folder for all the content and media of the sites in your network.
3. then you need to add some lines to your wp-config again and some lines to your .htaccess. as seen in this screenshot:
4. Now your regular WordPress installation is a Multi sites Network you can use as you used to work with your WordPress Mu. now we just have to wait and see if the upgrade from wordpress mu websites will go smooth as expected.