Create a network site for the member as part of membership to the main site.
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 a 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.