Telling Search Engines what pages you have with an XML sitemap
Adding an XML sitemap to the Google Search Console will help Google index and understand your site. This is a great first step to helping your site(s) be found for search. Make sure that you do this for both your portal site and your dealer sites as you create them.
You will want your dealer(s) to share their Google Search Console (formerly Google Webmaster Tools) with you so you can add this. Send them this link to make it easier.