The Invite Button is made available on the Profile page of every Binner. In order to invite a Binner, you need to visit to his profile page. There on the profile page, along with the page title "Binner : .....", the Invite Button is made available. Click on this button to proceed with sending invitation to the Profile owner.
This Invite Button will be visible only if, you are logged-in and you have not invited that Binner during last 15 days.