nodnarbo: No, I believe there are two concepts: referred users and referals (I thinks those are the terms in the english version).
- Referred users are just users you invited to the site, and register, doesn't matter if they become paid members or not. You can see them on the profile of the user who referred them (like in this list, for instance, where you can see one of the user's last action was at the same time of the first login so didn't even do anything, never played here).
- Referrals are when paid members that were invited by you eventually registered. These count for Brains.

Jaak: The first ones are guessed based on the link they followed when (s)he first came to the site. If that link has any reference to your userid, and (s)he register on the site, then that user will become one of your referred users. Probably when you invite them to a game, that link doesn't have your userId? You'll have to wait on a reply from Fencer on this one...