How to setup a site with multisite compatibility

I want to create a Directory website for my business club that come with a website for each of the business provider.

For your info, the directory website just consist of business listing & profile, i want to include the service for my user to create their own website using the multisite function in the same domain name as my primary multisite page

I have use the WP Multisite function + ProSites + Business Directory theme for it.

Now i'm stuck on how to integrate the registration for the multisite user dan the business directory.

Do you have any suggestion?