If a buyer already exists in your NuORDER contacts, you've already invited them to use NuORDER. When you add a contact to your portal with a buyer's email, they receive an invite email letting them know they've connected with a new brand.
If the buyer already has a NuORDER account but does not appear in your contacts, you can add their email to your contacts and they will receive an invite email letting them know they've connected with a new brand on NuORDER.