Inviting a supplier to join you on LeanLinking in order for you to communicate and share information more easily is very simple, but it is only something that can be done by Admin Users. As long as there is an e-mail address attached to the supplier (in the Point of Contact - Email field), any Admin User can invite the supplier from the Admin page.
Keep in mind that any linked suppliers will still only be able to see information that you have specifically chosen to share with them.