We usually put together a flyer letting people know when and where. We also let them know a little bit about what will go on (i.e, be ready to fish hard and walk. Please bring your own camping, fishing gear, and food.). Then we let the chips fall where they may. I think I mentioned that we had a few lean years but always a good time, you can't make people come. So, go with what is best for the core that has decided to put it together.
Set that date and let people know what to expect, that is about all you can do. Have fun though that is the important thing. I know that we have a blast at our camps every year.