Thank you everyone for your answers.
To be honest, I still don't know whether there is a "Only people you know in person" rule for giving invites to high-levelers, but if there is one somewhere, I'd be pretty surprised if they didn't check IP ranges from both the inviter and the invitee.
On the other hand, if there is no such rule, it's understandable that they don't and that they only care about how the invitee will behave once he registers.
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