Friend Inviter: positioned at Logout-screen when user leaves the website
I saw this implemented at a website (not XenForo) and I liked it very much.
The user clicks onto "Logout" and is in fact logged-out of the website.
The Logout-screen gives the standard-message like: "You are now logged-out. Thank you for visiting bla-bla-bla."
Underneath is a simple "Friend Inviter"-form which lets the user to invite friends to the website via "E-mail invitation".