Partners are usually POS agencies or resellers who are involved with the configuration of a dealer integration. They are added to the integration by navigating to the Partners tab under the Store ID. Click "Add" next to the Partner who needs to be associated with the dealer.
You can add as many Partners as necessary. You may also need to add these Partners to the Onboarding access for the dealer. This is explained on the Store ID - Onboarding Tab help page.