I would do two separate cards in the invitation, one being a more informal announcement and the other being the invite. The announcement could be something like, "Hannah and Scott are getting married! September 9, 2007, etc... just give the details. You can even add at the bottom in small print, "Ceremony held for family members only," or just add somewhere on the announcement "Hannah and Scott are getting married..... in the presence of their families." Then, you could do a formal invitation that says, "Your presence is requested at a reception to celebrate the marriage of Hannah Dee and Randall Scott at 6 o'clock in the evening, etc. (whatever the details are).

That's the best I can do for now. I think I have an invitation etiquette book somewhere so I may re-post later if I find something.