setting acceptance quota in an Outlook meeting request

A VP at the company that I work for wants to send out a meeting request to 80 people. However, he wants to set a quota of 50 positive responses to attend the meeting. In other words the first 50 responses saying that they will attend the meeting will be accepted. Anyone who responds after the first 50 responses accepting the meeting request will be told that the meeting is full. Does anyone know how to do this in Outlook 2003? All of our clients are Outlook 2003 and we are running Exchange 2003 as well.

Thanks for any suggestions!
Scott