WP-Invites was definitly the plugin I need for the website of a small association which want some private content on the site (visible only for registered members of the association) but also want to share this content with selected friends.
Together with "WP-Invites widget" plugin from Nettle, any member of the association can now invite some friends.
The website is using WP3 (yeah, just fresh new!) and Buddypress 220.127.116.11 and when I installed WP-Invites, it doesn't seem to do anything at all. I could see the invite code request in the register form, but I was able to validate the form without the invite code being checked.
In fact, it lacks a line at the end of the wp-invites.php (line 393 in my version) with the command :
add_action( 'bp_signup_validate', 'invites_validate_signup_fields',99,1);
With this, it works like a charm! Thanks to the devs.