This is a  Plus Add On.

Plus Add Ons are included in all Plus or higher premium membership plans.

How it Works

Using the Member Network Sites Add On, you can allow a member to purchase one or more sites as part of membership. Once configured, the member can purchase membership at the network’s main site (the primary domain of the network) and specify the “Site Name” and “Site Title” for their new site on the network.

The site will be created for them after registering. Any sites attached to a member will be deactivated when membership expires or cancels. If they sign up for a membership again, the site will be reactivated.

You can limit and pre-configure the themes and plugins available to the site created at checkout via the tutorial in this post.