Enabling/installing multisite with Communities

Is multisite a feature of WP that is built-in and needs to be enabled (as seen here https://premium.wpmudev.org/manuals/wpmu-manual-2/creating-a-network-to-enable-wordpress-multisite/) or is there a plugin called "multisite" that will manage this issue? I am getting the error Call to undefined function get_active_blog_for_user() whenever I try to do anything meaningful.