You can add a new supplier to site using both the webportal or the mobile app.
To add a supplier that has previously been added to your company to a specific site, follow the steps below:
Mobile
Ensure that you are checked in or have selected the site you are adding the supplier to in the site selection dropdown. Then navigate to the Host Panel and select Supplier.
Use the 'Add Supplier to Site' button, select the dropdown and pick a supplier from the company list.
To add a supplier that is not on the company list, use the dropdown and select --- Add new supplier to site ---. To ensure there are no duplicates, check the dropdown first to make sure the supplier hasn't already been added.
Webportal
Navigate to Sites in the left navigation column and choose which site you'd like to add the supplier to. Select the Suppliers button.
In the List of Site Suppliers, select + New 
to add a new supplier to site.
Choose or search from the dropdown by typing in the name and click Submit 
.