Recently I have worked with OpenInviter to work with PunBB. OpenInviter already has a plugin (mod) that enables it to work with PunBB. But Idid some more works that will enable PunBB forum admins to tracks how many users have been invited, which users has invited, when invited, when the invited friends visited the site and when they registered (if they did). So, it will be easier for the admins to manage invitations. Admins can even award the top referrals.
But this script or OpenInviter do not store the importer’s (user’s) email login details. So you can use it safely.
When someone visits the site upon getting invitations from forum user, a cookie is set in his computer so that he can be tracked if in future (untils 6month s or deleted) he directly visits the site.
I have worked with PunBB 1.2.x. But I don’t have any plan to release this script for 1.2x because manual source hacking, which I dislike. I wish to release as an extension for PunBB 1.3.x. But if someone extreamly need this modifcation, please let me know. I will try to deliver to you.