Patrice's idea of offering a discount would be a great way to open the conversation - "i didn't think to mention it the other day, but if you attend with someone you've referred you get a 10% discount!". If she indicates she won't be paying that's your cue to say you're sorry, but you can't allow that because others have taken advantage in the past and you've had to put a stop to it. That way you can bring it up in a positive way and address the "but others have done. it" issue up front, while blaming your new rule on the others who have taken advantage. Let us know how it works out!