I saw this posted on another board. It is supposedly from Emily Post but I didn't find the original posting:
Q. How do you send an invitation to about 10 people for a small lunch get together just to celebrate a friend having her 4th child (not an official shower, just a little intimate time as friends)? Also, how do I let the people know that we'll all be paying for our own lunch?
A. "Encore showers" can be fun for both the expectant mother and her friends. These showers or
the small luncheon you describe should be limited to close relatives and very close friends. Due to the
intimacy of the event, there is no need to send invitations. In addition, written invitations are not sent to
guests for an event where they are expected to pay their own way.
Instead, make it a group effort and make it clear that you are only the organizer, not the hostess. In this case, the phone works best. "Sally, a bunch of us were thinking of treating Janie to lunch before the baby arrives. It will be at noon at Chez Harry on August 4th. It's Dutch treat (or, we are splitting the bill), and we will all split Janie's meal." You don't want anyone to be surprised at the event, so make sure they are aware of the payment arrangements ahead of time.