You have created your market and now you want to invite vendors to join your market. Its very easy. Just tell your vendors to follow these steps.
Step 1 : Vendor would download Nearby Shops from Google Play
Step 2 : Vendor would Select the Given Market
Step 3 : Vendor would Login to the app
Step 4 : Vendor would click "Become a Seller" option on Market Screen / Profile Screen
Step 5 : Vendor would fill Shop information and Click on Add Button
Step 6 : Market admin would Enable the Shop from his Admin Dashboard
As a market admin unless you dont approve / enable a shop it wont be visible to the customers. This is done to ensure that market admins can enforce rules and regulations.
After all of these steps are complete the vendor would also need to setup his shop and make it ready to receive orders. The instructions to setup your shop is given in another tutorial.