How to Add Existing Users in bulk?

I have the Responsive Classifieds theme installed in a subfolder on my site at https://ehangar.com/classifieds

With this setup, I have discovered that members registered on the main site at https://ehangar.com can access the Classifieds subfolder section and see themselves as logged in, and can access the Members Area and can also create listings.

However, these listings cannot be displayed until I have manually added the member under the wp-admin area of the Classifieds section.

I have over 1,600 registered members in the main site. How do I add them all in bulk to the Classifieds section so that they can post listings?