Be sure to post in either the Kitchen Table forum or the Cooking forum. There should be lots of good advice there,

http://ths.gardenweb.com/forums/kitchentable/

Don't panic! It will be just fine.

If you're talking about what to do about food, I'm thinking you can probably still cook pretty much the same menu; if it isn't raining too hard, you can probably even still use the grill.

If you are asking about activities -- well, what would you do if you were outdoors? If you were going to sit around and chat, then just do that indoors. If you were going to play games, then you've got a group that likes games, so you can play indoor games instead, like charades. If all else fails, watch a movie!

Have fun. I know your guests will.

Feed then food....serve them drinks. Let the young ones sit on the floor, set up a card table or 2. People will do the same as they did outside....but inside.

How did it go? Great, I bet!